Send any picture you want immortalized by one of Crappy Portraits terrible artists. If you can't decide, send them a couple to choose from. All they ask is that you please try to keep the overall file size under 5 megabytes.
You don't have to, but they highly recommend that you tell them something about you or whoever else is in the photo. It can be a list of your favorite cheeses, a tale about the ever-elusive Sasquatch, or a haiku about roller disco - they aren't picky. Just keep in mind that you'll probably get a better portrait if you tell them a bit about yourself.
And they know you have lots of friends or family and might want to send them photos with giant groups of people. To be honest, none of them really like drawing groups of more than 6 people. They'll eventually get to them, but they tend to get pushed to the side. Why not just tell your friends or family about them instead and have them get their own portraits?
Also, they don't mind making your photo link to your blog, your favorite cause, or whatever you like. Make sure to include it in your submission.