When I updated the subject/description of a MUC the other day, I realized that doing so removes the avatar picture from the MUC. I just verified again:

Before changing the subject:

After changing the subject:

Please note the change in subject "hookipa.net & xmpp.social" -> "hookipa.net / xmpp.social". Please also note that you need to open the complete settings. Just changing the subject in the small windows does work, though

Here's a video of this:

https://user-images.githubusercontent.com/18173881/128703027-e3812366-140f-4371-bab1-ed36b8d027cb.mp4

Server is ejabberd 21.01-2~bpo10+1 on Debian, MUC is on chat.xmpp.social.