Go into "My Pictures" on the left hand menu in your Profile, and set whichever picture you want as your profile picture to the default.

I'm guessing you already did this, though. The profile pictures actually take a little while to change from one to another - they don't change immediately after you physically change the default picture. Just give it some time, and it'll change.

Gunpowder: FUCKING ROCKS!!!
Quote by The Madcap
[witty set-up]
Gunpowder FUCKING ROCKS!!!!!

Quote by Kensai

Gunpowder you fucking rock!!

Quote by Dirge Humani
Now I can say, with sufficient certainly, that you, Gunpowder...