This is an interesting issue, and though I don't have a solution, I'd like to know anything I could about these group pictures in Moodle 2.0. Where one creates a group for assigning students, there is an opportunity to upload a picture. I've tried that, and then allocated sample students to the group/team. In fact I was able to assign a student to two such within-class teams (I think cohorts may be 'teams' composed of students within the site but across classes?)
I have no idea where this group photos appear, though I'd love for teams to have a home page or forum for shared activities, or publishing wikis to the site learning community. I've not yet stumbled on to many uses of this image.
I did note that when a student posts to a forum to which others are subscribed, the subscribed student gets a copy of email which is pretty html intensive and includes a photo from the author's profile. In my experiments, the student who I had assigned to two groups also had forum posts which produced "crashed graphics" which I think may be poorly pathed thumbnails to the images for groups that the author is a member of? I've left a post in the general problems area hoping that someone familiar with Moodle 2.0 group images and email templates might be able to help.