I think if you download something here under a particular license then you have to agree to that license, if the file has a different license elsewhere then it is usually picked up and removed for licensing issues, however the odd one may slip the net. As long as you can prove the file you use is accompanied by the license and you adhere to the license you haven't done anything wrong really, however the author of the file, or user that submitted the file in the first place may have committed an licensing offence, so it should be taken up with them for legalities. 

Example: if you have 2 of the same artwork, one is public domain and one is protected.

you download the public domain and use, that's fine, hey it's public domain right?, but someone has recognised that it's there work and is protected, so they jump up and down on you, you say hey but I got this here with this license, they say ok, we must investigate this, so you have to take down you project whilst we find out who uploaded the original artwork. Then they should be liable really I think, so you have to find something else to use, complete bummer. Not sure what happens though if the uploader Cannot be found, long as you can prove the license you got with the file.

thats my understanding anyway, anyone feel free to correct my opinion.:)

You're overthinking it.  Unless you have evidence of plagiarism or copyright infringement, you can make the fair assumption that what something appears to be is what it really is.  To do otherwise will put you (and any company) into constant paralysis over legal paranoia.

The "stories" you've heard are almost certainly only that -- fiction.  "Immensely high" compensation over non-commercial use is incredibly rare.  I'd be shocked if you could offer even one verifiable example of that.

Further, everything is derivative of something (even if only the natural world).  Copyright is obsessed with dichotomies like "idea versus expression" and "original versus derivative", but the reality is that all dichotomies are oversimplifications at best and outright false at worst.  That's why asking a question like "are you really sure you didn't copy anything" is utterly pointless.  Everyone copies, always, all the time, whether they realize it or not.  That's the very nature of information.

Assuming you have actual serious questions about copyright or IP law generally, ask a lawyer (preferably in meat space).  The internet is one of the worst places to gather information about serious topics like law or medicine, and it gets worse about that with every passing year due to ever-increasing homogenization and commercialization.  (See also the thread at http://opengameart.org/forumtopic/fan-based-games where I just posted a rebuttal to a long stream of nonsense.)

The best I can do is leave a "flame" comment because I don't have a "Flag for copyright violation" button.  All I can do is either that or report spam.  The only times they actually get dealt with is if said plagiarist has a temper tantrum for the staff to notice there's something wrong with the asset, or if Snabisch doens't bury the recent post history with sugar filler to every submission, congratulating even 3d models submitted as only picture files. Many obvious infractions i've pointed out months and years ago are still here, including the Earthbound and DragonballZ characters

Relatedly in the context of Free content development, version control systems don't work in favor of removing infractions, because it'll be in the history, and revising hsitory to cleanse copyrighted material is a big no-no to "Freedom", etc.  I had to retract a big release in the past because some mapper insisted some textures were Free but were really NC from CGtextures. Since then i've had a steady stream of sexist/racist slurs, "feminazi", etc. headed my way and no more contributors.

@powertomato :

Ah, you're talking specifically about copyright trolls.  Yeah, they exist.  The reason they ask for 500 to 5000 euros (which is not "immensely high" by any reasonable standard) is because they know some fraction of the recipients will pay it blindly and unquestionably.  The companies that do this trolling typically don't own the copyright, and in rare cases they've been found to not even have been authorized by the actual copyright holder (well, either that, or the holder publically lied about the authorization to save face).

Probably the most important thing to understand about copyright trolls is that there's no guarantee they won't sue (or even go away) if you pay up.  It's essentially an extortion racket for them, so they have no incentive to stop threatening you if they believe they can get even more money.  Eventually the courts (and smart defense lawyers) wisen up to the abusive tactics and stop them, but it typically takes years, even a decade to finally get sanctions and shut them down.  Unfortunately, there's no real way to prevent them from starting up again under a new identity and new middlemen.  Actual criminal punishment even for egregiously obvious abuse of IP is absurdly rare -- I can only think of a couple cases off-hand in the United States (and those were ultimately for perjury).

As for artists not knowing copyright law, well ... you're right.  They don't.  Almost nobody does; not even most lawyers know it.  Unfortunately, they can't really be expected to know it, either, because it's complex and subjective.  (If you don't believe that, try reading through court case files sometime.)  What counts as an infringement in one country or one court can easily be non-infringing at another place or time.  The very standards themselves have actually changed a lot since the rise of the internet, whether we realize it or not.  For example, it's nearly unthinkable that the many thousands of advertising-laden (commercial) Let's Play videos would have been permitted back in the 80s on TV.  The statutes haven't changed much in most countries, though.  What changed was (a) people's interpretation of what qualified as "criticism" or "commentary" for fair use purposes and (b) new companies emerged who could profit from new business models that required a more flexible understanding of copyright.

Regarding sprite rips specifically, I'm pretty sure they're just trying to influence where they get uploaded (in terms of the major sites) and whether they get any credit for the rip.  It's pretty broadly understood that they have no copyrightable expression in the rips, whether they explicitly state that or not.  As for laypeople, admittedly there are some out there ignorant enough to blithely reuse whatever they find on the internet regardless of context or facts.  The ultimate reason for that is because they tend to get away with it in the vast, vast majority of cases (as they should -- nobody wants to live in the totalitarian hellscape where every copying violation is caught and punished).  So much so that certain communities seem to have clubs of people who take no greater joy in the world than shaming reuse of copyrighted materials.  Including, ironically enough, even inside communities like the sprite rippers where they've all too obviously extracted and reused material in a manner unauthorized by the IP holder.

As for any OGA violations you find, you'll have to keep trying to get the attention of a moderator or admin who actually has the power to flag it.  That particular item probably should have been purged from the start the first time Surt identified something suspicious about it.  In general, I stay away from random users like that who suddenly post a bunch of stuff in inconsistent styles.  When you have unanswered doubts about the origins of something, it's usually faster to just inquire.  If you can't find contact information, they don't answer, or the answer is unsatisfying, then just don't use it and move on to something else.  Life is too short to spend hours every day worried about this kind of issue, especially when it's not your job and you're not earning anything for it.

undesired: "I don't have a "Flag for copyright violation" button.  All I can do is either that or report spam."

Click "Report spam". I actually do review those. It is the preferred way to draw attention to assets with copyright issues. It helps if you comment with details of the infringement, too, but just a comment alone often goes unnoticed. When I say a comment is helpful, I mean something more than just "this is suspiciously similar to a copyrighted work". Give details, evidence, screenshots, links, comparisons, something. I don't know everything about every IP, so it helps having all you-guys's expertise added in. :)

There is a lot of older stuff that I don't know about that I'm sure people have drawn attention to, but I'm a relatively new moderator, so I wouldn't have seen those complaints at the time. For others, the potential infringement is unclear or borderline, not obvious. For example http://opengameart.org/content/elbaun has some substantial difference and little indication of adaptation from the Ness character beyond the clothing coloring, so I decided to leave it alone until someone could provide more information.

powertomato: "Even on OGA various non-free material was released without anyone noticing for months."

That's true. Sometimes stuff gets missed. Don't be afraid to draw attention to it. Mark it as spam so the moderators notice it. It doesn't guarantee it will get removed, though. Sometimes a moderator decides it isn't a violation. Other times they just continue to miss it. If you think something isn't getting the attention it needs even after marking it, PM me about it.

kagerato: "As for any OGA violations you find, you'll have to keep trying to get the attention of a moderator or admin who actually has the power to flag it."

Yup. Report spam. :)

REPORT SPAM!

CLICK IT!