I never actually fapped to any of these pics
Right there with you, because most of them were actually pretty shit.
The last time I jerked to stills was when they were passed around as stacks of floppy disks.

Don't really understand the turn on either
There's a moment of "whoa so that's what she looks like naked" but other than that... 
Voyeurism, only not frowned on as widely for some reason.