Sorry if this isn't the appropriate place to report code_swarm issues. This fork is more active than the mainline on Google Code so I figure I'll report here.
When visualizing a large repository where most of the committers don't have gravatars, I find its less clutter to have a blank default avatar rather than using an identicon or similar. See http://schwern.net/tmp/perl5.avi for an example.
I tried a 40x40 transparent png and that showed up as a white square. A 1x1 transparent pixel showed up as a dot. A 1x1 black pixel was the least obtrusive, but it doesn't work with circular avatars (next bug).
It would be nice if either A) avatar transparency was preserved or B) you could turn off default avatars. A would be nice in general.