There is a question as to whether users' images should have meaningful alt text that conveys the content of the image, in particular when people use images of e.g. their pet or baby or some other icon. These images help to build a sense of community and convey something about the person which should perhaps be conveyed to blind users.
Add one field to user profile so user can edit this. If it's blank, then use the user's name in display "Picture of Kim Bloggs"