Rules are simple.
Credit me by linking to the stock photo, either in your deviation description if it's here on DA or in the standard creator's note/bottom of your personal page if it is hosted elsewhere.
My curiosity would appreciate it if you left a link for me so I can see what you've done with my stuff.
You don't have to ask to use anything of mine. You can use my stock for prints, for school projects, for pictures that are uploaded to social networking sites, for backgrounds, for personal sites. You DON'T HAVE TO ASK.
There, simple, because I know that's how you like it. :)