1)  Face pics, for me, are almost a neccessity.  It has to be a pretty hot picture otherwise for me to want to cum on it.  If you're not into putting your face out there, I'm sure you'll find lots of other guys willing to cum on your pics.

2) It should be a good quality pic!  A nice clear resolution is key, and the size of about your average computer desktop is nice.

3) There are also lots of other things that a pervert like me will get an extra kick out of.  A picture of you with a girlfriend who I imagine knows nothing about this is extra hot.  I don't feel like explaining why that is at the moment.  Lets just say that it adds to the soup.  Certain poses are sexy.  I like more submissive-looking poses (ie looking up at the camera while sitting).

4) Regular old pictures copied from your facebook do just nicely!  No need for fancy slutty poses.  I like to remain in the fantasy that you're externally a really nice girl who is secretly an extra nasty slut inside!

5) Preferably you!  Or at least make a convincing lie that it is you or it could very well ruin the mood.  Part of the fun is in the challenge of finding a woman who loves this as much as I do.  Remember, I could go anywhere on the net looking for pics that (usually dudes) put of people they don't even know and that's not challenging at all!