I think this might be possible. Open up "Contacts" to get the "All Contacts" view. Click the menu in the title bar, and then the "My Information" button. It'll show all the info that the phone has stored for you. Clicking the photo there will let you change your photo like you can for any other contact.

I haven't tested that this works with IM etc. yet, but at a guess it might do.

EDIT: Actually, this doesn't work, but setting your avatar image per-IM connection does send the avatar and display it as you'd expect.