In answer to your first question, it is sometimes appropriate to leave alt text empty. The user icon is (I believe) one of those cases, as the user's name is the only useful textual replacement for the image, and the icon is usually accompanied by that information anyway.
The entire subject of accessible images and various related concerns are covered in depth by Chapter 6 of Joe Clark's Building Accessible Websites, which you can read on-line.
In particular the section about half-way down entitled Identical alt and subsequent words covers the situation of user icons..
Warning: I believe there is a reference and link to an adult site in that chapter.