Mine is the cover of The Beatles album Let It Be.
My favorite group of all time by far. I can listen to Lennon all day. My favorite album was The White Album...but it'd be hard to use as an avatar. It'd just be small and white. Let It Be is my 2nd favorite album by them.
Edit: Let It Be was my favorite album cover though. I thought it was great how they showed that they were still a group, but very much individuals going there own way by this point in the group. This was highlighted by 4 seperate pictures put close together. Brilliant artists, although egos got in the way.