I'm sure that the original owners/creators retain the copyright, just because they're posted on the internet doesn't mean you can do whaever you want with them. If you've got something in mind for a specific image, why don' you email the poster and ask if you can use it.

It would be kind of cool if we could specify the level and kind of copyright protection for gallery images, I'm thinking of the way flickr integrates Creative Commons licenses ...

As I understand it, all works created in the US (at least) after 1989 are given copyright protection automatically.

Well I dont see any copyright signs on or around the images, and I will be crediting the names of the people who submitted the image.

The images are copyrighted even if not so identified. Giving credit is a good idea -- the least that one should do. That's what I'd expect in the circumstance.