I don't know if you mean panties being pulled down to expose a lady's pussy / ass, which is an interesting idea, or if you mean for spanking purposes, but, here are two pictures that you could use a little imagination on to pretend it's both.
They might get taken down for being professional photos. After all, people own the RIGHTS to pictures of a lady's panties or bottoms being pulled down, pussy exposed, etc.
http://www.herkinkyporn.com/wp-conte.../abcdefg44.jpg
http://red.blogtur.com/uploads/t/tym...gp_set7_03.jpg
two pictures I found by searching "panties pulled down to knees" on Google.
Bikini bottoms pulled down to knees might also be a good search.
I also like those bra-n-panties wrestling matches, wish there was one for full 100% female nudity, ripping off a bikini.
There is a video online for free, involving strip wrestling, which involves one lady yelling "You cheated!" and ripping off the other girl's two-piece bikini, including the bottoms, pulled down from the front side, but, it is censored and blurred. That video was so hot, though. It still makes me . . . wistful.