I changed my profile picture to a picture from a party with a friend and this one girl. She commented that she likes my picture, and was wondering whats the next best step?

I think you're over-analyzing. But, you want to answer back with something that won't kill a conversation before it even starts...A simple thanks will do that.

If she looks like she's got a high self-esteem hit her with a neg about her picture, be playful build rapport, be c/f. If you think she doesn't, drop the neg.
Ex.
HB: nice picture

YOU: Thanks, wish I could say the same thing about yours!

HB: hahaha blah blah blah (you're so mean!)

YOU: hey just saying! if you want a better profile picture call me and i'll see if i can free up some time to take a couple pictures with you

HB: hahahaha blah blah blah

etc.

It should probably be a little more natural and less forced than the stuff I said...Just go with your gut. Say things that result in responses. if you fail to come off as funny in the c/f or the neg, you will be burned very bad.