Almost every photo on the net is copyright and so illegal to copy and use elsewhere without the permission of the copyright owner; usually the photographer or the person who commissioned him/her.
Now, personally I'm always flattered if someone wishes to use one of my pictures. I get a bit peeved if they do so without asking me and without crediting it to me. However, if you ask to use one of mine for any non-commercial purpose I'm almost certain to say go ahead.
On this site, if you want to use a photo, just ask the photographer and accept his answer.