What license to use?

I discovered a photo sharing site called 5000 Free Stock Photos. You can for more resources here It contains free photos that you can use, even for commercial use.

Since I'm not a professional photographer, artist, or musician I would just let my work be part of the public domain. I have lots of pictures that I have uploaded to different sites such as MySpace, Facebook, Flickr, Slide, Photobucket, etc. and I don't mind if others use them.

If I were a professional and I was just starting out then I might allow some of my work to be in the public domain. However, if I needed the money really bad or if I had spent a lot of time, effort, and money to create my piece, I would copyright the work.

One problem that I see with public domain photos and other media is that one day the creator could wake up and decide to copyright it. So my question is, say you have their work on a commercial website or in a presentation and they find out you have it even though you obtained it before the copyright can they still take legal action and win? I would be very careful about using public domain work.

I have thought about copyrighting some of my photos if I ever include them on one of my websites but as far as the regular photos I have on Web 2.0 sharing sites I don't feel the need to copyright them. If others like them and want to use them, well great!