Thank for you post. Its good that you are experimenting from your mobile browser.
As the previous user posted,
Click 'Full' down at the bottom of the page
where it say 'Version: Mobile | Full' and then under your 'My Community Settings', one can change the Avatar... from iPhone browser
If you want to upload actual photograph to associate with your profile, you might have to do it from a PC, MacBook or certain mobile OSs am afraid..
iPhone Sandboxes the mobile Safari browser.. Hence you will not be able to upload or download image (any file for that matter) directly from the mobile browser..