Prologue anonymous comment avatar broken icon image

I installed prologue 1.3 on WP 2.5.1 and cannot figure out why comments posted by anonymous user comments are showing up with the broken icon image in IE. (a blank box shows up in Firefox) Registered users get their Gravatars or the default Gravatar image. I would like to just use a default icon image for anonymous users but I cannot figure out what I need to change in the comments.php file. I would appreciate someones help with the code, Thanks!