1 - Click on UserCP on the top menu of vBulletin
2 - Select "Edit Avatar" on the menu
3 - Under "Custom Avatar Section" check "Use Custom Avatar"
4 - Type in a URL to the avatar you want to use in the space provided by Option 1.
5 - Or use the browse button to the right of the space provided by Option 2 to load a graphics from you computer.
Note: vBulletin suggests using avatar graphics no larger than 80 x 80 for best results.