And why is this here? Nothing about the picture is funny.

I'm guessing that you're not aware that the metal cabinet on the wall with the door on it is called a "network rack" (in this case a 6U rack but that isn't needed info for the joke).

The pun in the title is the kind of thing you’d hear coming from the creep in the office who thinks he’s funny but is actually the opposite of funny.

So part of humor is subverting expectations. The punchline in the title (not a pun BTW, a pun would be different word that sounds the same) would be exactly what you thought, a crude joke referring to women's breasts, however when you open the picture, you're greeted with a network rack..ha ha (expectations subverted), but upon closer inspection there is indeed a picture of a woman's breasts and the original crude punchline is the indeed what is meant, (expectations subverted again). So the humor isn't the crude joke, but that you thought it was the crude joke, then it wasn't, then it was again. That's humorous except when you have to explain it to someone.