It's probably because they've seen what you look like and that your so ugly that they grab the worst pictures avaliable and post those so you won't bother them.
I disagree. The worst part about them is that most of them are as shallow as a pot hole after a light drizzle. Had a few buddies who tried these things out while we were in college. They told me of all the unreal expectations that women on these sites have for snagging a dude. As if guys who look like Brad Pitt and have a six figure salary to match have problems landing girls