Surprisingly enough, all photos are copyright to their owners under US federal law.
http://www.photolaw.net/faq.html
For example, let's say someone wants to post a picture of themself or with permission a partner, the person who owns that photo and the right to the image, holds the copyright, even if they don't mark the photo as such.
A good example, is Barton's recent posting of Kat with her new sexy collar. Barton and Kat are the copyright holders of that photo. Just because it was posted on the web, doesn't mean it is fair game to be copied, distributed, etc.
Rabbit, are you wanting any member to contact you before posting any personal picture? Or are you specifically calling out images where a (c) copyright to someone has written on the photo, or it's clearly artwork that belongs to someone other than the poster - like say a well known cartoonist's photos or images from say artmam?