If your goal is to be as sketchy as possible.
*Don't smile. Smiling indicates that you are flippant, whereas you want to project to any potential dates out there that you are SERIOUS about dating.
*Blank out your eyes. Everyone knows that the eyes are the window to the soul. If you're a total jerk, cheater, or just shy, you're going to want to prevent your suitor from seeing your soul for as long as possible. On the actual date, consider going somewhere appropriate for sunglasses or a Geordie (Star Trek Next Gen)-style visor.
*If a former partner is in the picture, edit them out. Using the Eraser in Paint to scribble over his/her face, using a large black square to cover him or her, and cropping out up to 70% of the picture are all popular methods of removing a former loved one. Because heaven forbid your future date might see you have a romantic past!!
*Stand next to a sportscar. This will not make you look like a tool at ALL. Especially if the car isn't yours. Nothing that attracts people looking for a relationship more than saying "I already have something I consider more important than you."
*Hold a baby. This won't make you look like a pedophile at ALL. Especially if the baby isn't yours.
*Show off as much muscle as possible. Don't forget that dating is a contest and you want to WIN. Showing weakness isn't going to get you anywhere, and the best way to avoid appearing to be weak is to make it amply clear that you are STRONG. Forget the fact the women at your gym seem to be more repelled than attracted by your grunting reps and giant, sweaty neck and fingers thick as sausages. This is an ACHIEVEMENT, dammit!
*Be a hyperbole. A picture is worth a thousand words, so why bother filling out your profile if you can just pictures that express everything about you that you would write out if you weren't too lazy to type? If you're into hip hop, wear your newest baseball hat and your shiniest bling. Sports people, make sure that you're wearing some sort of "gear" in every picture, including the appropriate obnoxiously huge, polarized goggles (bonus- also fulfills rule #1!). For those who like to travel, each picture should be taken on a different continent, giving the impression that you're jetting off to a new exotic place approximately every six minutes.
And finally, if all else fails, Instill a sense of mystery. Whether you're shy about your looks or a married individual avoiding identification, your ace in the hole is the cryptic art shot. Out of focus, seeped in shadow, or hidden by a motorcycle helmet, this look works for just about everybody. Another popular style is the half-face shot (vertical OR horizontal cut), which hides anything from double chins to an abnormally runny nostril until it's too late. Even those seasoned in picking dates online can forget the dangers of this little trick, because in the minority of cases that artistic choice might not be hiding anything at all.