Your avatar must be *.gif, *.jpg, or *.png. No *.bmp pictures work. Judging by the format of your sig, this seems to be the case. If you don't know how to change the format of the picture, perhaps you can go to
Open Requests, where someone will probably be willing to help you.
I'll lock this, as there's already an
existing topic for avatar problems in PPT Discussion and Questions.