I am not a lawyer, etc., etc.
I don’t have any where off the top of my head, but such does exist. Try http://search.creativecommons.org/ for Creative Commons-licensed material. Note that the terms of the licenses differ as to whether you are allowed to create derivative works (alter the material), include it in commercial products, and whether you might be required to provide attribution (publicly acknowledge “jayjay32 did the awesome background image”).
You should get up to speed on those laws before you get burned. O’Reilly has a great free book on intellectual property issues: http://oreilly.com/openbook/osfreesoft/book/index.html. I recommend you read the preface and first chapter before publishing your app, as a lot of code and other resources you find will be affected by these issues. The last two chapters are also relevant. The chapters in the middle go in-depth on the major licenses you’ll encounter in the wild.
Only if it says it is, otherwise, nope. Even then, there might be conditions attached. For example, the common BSD software license says, roughly, “go ahead and get wild, but keep this license and publicly acknowledge you used this software by these handsome fellows”. If you want to see someone get really wild and crazy with the conditions you must satisfy in order to legally use something per copyright law, check out any of the GNU licenses, like the AGPLv3.
No, it’s considered a derivative work.