Butt (hue) pull your pants up first.

I totally agree with you except that the fml is super vague, so my second thought is: What if OP is that kind of person that has their butt crack hanging out all the time when they're sitting or standing? Like that Comic-Con meme and the dude who'd pose next to painstakingly butts that display full, half, or crescent moons. OP also didn't specify that you could see that it was THEM (i.e., their face isn't showing or something that's obviously them) but perhaps OP recognized it to be themself and perhaps only those that know OP might recognize it to be them too. This doesn't entirely mean that the photographer was being malicious our had such intentions, necessarily. Especially if they discreetly put it on OP's desk as to bring awareness to OP anonymously. I would still recommend OP go to HR if they feel unfairly harassed or exploited. Now this is all speculation and I'm definitely on a FYL side of things but if my theory holds any truth then I can also see it be a big YDI. Like how do you not feel a cold wind lightly kiss your exposed buttocks in the first place? I'm not assuming OP is one gender (all OP gender also doesn't appear on my phone for some reason?) but nonetheless, male, female, other and/or banana, not everyone wants to look at a crack ass all day, especially if this is a normal clothing style choice of OP.

What makes you think that the HR would side with the OP? If it was indeed the OP's exposed butt crack then they could possibly get in trouble for not wearing appropriate attire to work. Take it as a warning to cover up from now on.

I was waiting for someone else to say that! Everyones talking about how inconsiderate the picture taker is, how about OP bring inconsiderate for constantly showing their ass(if someone went out of their way to take a picture, I'm sure it wasn't the first time they saw it)???

Still, I'd rather not someone have a picture of my ass crack in their phone, especially if you could see anything identifying. They could have just written a note to say "your ass crack is showing", or printed it if worried about them recognizing the handwriting. Would be an interesting thing to find in the print queue though. XD

I did not consider the option of OP being sloppy by having their crack hanging out. However, it is still a problem that instead of the person speaking to a manager or to OP directly, they took a photo of them and placed it on their desk. Seems immature or creepy to me. But I've been on the receiving end of harassment at work before so maybe my pov is a little different.

(Why are you being down-voted...? :c..) Exactly. It could EASILY be a socially-awkward shy coworker doing exactly this....

Edit: not being downvoted anymore, I see! :D Also wanted to point out that it could be someone who was worried about it being seen as sexual harassment, so they told you anonymously...?

I dont