Yes, you can do so by going to: http://twitter.com/settings/profile, and clicking on "change image."
Go to settings>Profile and change the profile pic.
Change the pfp on your browser.
You can't you need to use the web version.
The Avi on Twitter is your profile picture (Avitar).
if you do not have a picture on twitter that is what comes up as your profile pic.
Its basically your twitter "profile picture"
Clicking on the picture on their profile will open a larger version unless their profile is protected.
You go to edit profile, click browse and choose a picture from your computer .
@justinbieber
Profile pictures on Twitter are usually refered to as 'icons', although there is no official name for them.
To change your profile picture on wikianswers go to your profile and upload it from there.
You click on upload a picture!