I searched and found an older, closed discussion: https://moodle.org/mod/forum/discuss.php?d=349205
I tried experimenting with the CSS, but was unable to make the group pictures display horizontally. But I'm using Boost on MoodleCloud, rather than the older Essential theme, so there are different divs and such. (As of this writing, MoodleCloud is on 3.5.)
At minimum, I need to make the images smaller-- they're displaying at 48px, and we have a lot of groups on this site, so we use much smaller images (20px). But we need a more compact format to avoid making every forum post stretch to include all the images. I'd actually prefer 3 columns with the smaller image size in the current location, but a single horizontal row would also work.
Any suggestions? Thanks!