Hmm. It was a pretty instinctive guess, but let me reason it out.

Those pictures may have been genuine romantic encounters. They may not. I don't know the person. But the pictures in question are mostly of a variety I have seen countless times: the "we'll kiss each other in front of the camera but we're looking right at it to make sure we're being seen" variety. The sort done for cheap thrills in front of an audience, particularly a camera.

If there were pictures of her holding hands with a girlfriend, or with her arms around one in a way that isn't reflected in her pictures with guys, I might estimate otherwise. But there really isn't much other than the party pictures. We see pictures with her and some guys outside party settings, several of which look like they are about to go on a date, as a couple. Some even look like earlier prom pictures.

Also, the main reason i don't think the quantity of pictures matters is that repetition does not connote sincerity-- you may ask every person in your office how they're doing when you walk into work in the morning, but that doesn't mean you want to hear an answer other than "fine, and yourself?" The remains just as true for the first person you ask it of as the last... Of course, it depends on your workplace. Some places I've worked, I'd want to hear the answer. Some places, not so much. But overall, it's just a greeting. The phrase "How are you doing?" doesn't always indicate the words literally spoken, just as the action of kissing a person doesn't indicate romantic feelings.

If she routinely kisses girls at parties to liven up the parties, it might just mean she goes to lots of parties.