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.