What sort of copyright laws do I need to be aware of if I want to host an image posting site?

First thing you need to know is that if someone uploads something that's copyrighted and that person didn't have permission, you can be responsible for copyright violation. You need a clear policy for your users that they are not uploaded anything for which they do not own the copyright or which is not in the public domain. You should have a mechanism for copyright holders to easily notify you of violations. When you are notified, you should remove the offending material until proof can be provided from your user that they do, in fact, own the copyright or have license.