If I have a blog and post up a picture of some artists work, he/she can come after me for a copyright breach. However what happens on a large website where users can upload images as they wish. Moderators can't be educated on whether every user has permission to publish that image. In the event that a user posts a copyright image that they do have have permission for on a community forum, how to the website owners protect themselves from a claim?
In the USA, you are covered by the DMCA act, which you should most definitely read.
I can't sue you and your website for copyright infringement unless I first send you a DMCA takedown notice. (Of course I can sue, but I will lose).
In the DMCA notice I have to tell you who I am so that you can contact me, I have to tell you under perjury that I am the copyright holder or an agent of the copyright holder of some work, and that I believe your website is infringing on that copyright. You then have the choice to remove the material, which means I cannot sue you for copyright infringement because you acted on my DMCA notice, or you can refuse to remove it and I can include you in a copyright infringement lawsuit. By not acting on a DMCA notice you take full responsibility.
If you remove the material, you should inform the person who uploaded it. That person can decide to be Ok with the removal (and hope they won't get sued for copyright infringement, and they usually will be fine), or they can send you a counter notice. That counter notice would tell you that the uploader believes he or she isn't infringing any copyright. After receiving a counter notice, you may reinstate the material, and you tell the sender of the DMCA notice. Again, you are now legally protected. The uploader can now be quite sure to be sued, unless the DMCA notice was sent in error.
In the US, If the reposted content is protected by Fair Use, there's no problem.
Anything not covered by Fair Use is a violation of copyright. In the US, the 3rd party website or forum only has liability once they are notified of the infringement if they don't take action to remove the content.
- In general, any commercial entity or even presumably a major nonprofit forum is going to get liability insurance to protect against this.
Usually a "Cease and Desist" letter precedes any legal action. The reason for this is intellectual property litigation tends to be "ruinously expensive", and thus is avoided even by most major companies if other options provide remedy.
Where you do see large companies or individuals pursuing litigation is in instances of flagrant violation (like the old file sharing sites.) Sometimes artists will pursue such a suit to bar someone they don't like from utilizing their art, such as in the case of political campaigns using popular songs.
But it's much more rare to see a case go all the way to a decision. Large sites like youtube have a history of "erring on the side of caution" and removing content even when it's not clear it is truly in violation. (This likely has to do with the 3x damages. The person who posted isn't a lucrative target, but a big company definitely is.)