This is not wpForo core related issue. The problem may come from, UM, Lazy Load, optimizing plugin from the theme or another plugin in your website, you have to find out where is the problem coming from.
WpForo has 3 avatar variants, if default avatar is selected wpForo uses WordPress avatar system so if the issue on WordPress will be solved wpForo will show the avatars correctly.
How you see in the screenshots below, my avatar is showing correctly, that's because I have added the avatar by wpForo account.
- This question was added on when I asked in a reply how you get the initials in a circle by default when the user has not supplied their own Avatar. You said that the Leira Letter plugin did it. I installed this plugin, but my forums still show an empty circle for users that have not specified an Avatar for their account. So what am I doing wrong here?
also enabled here?
Wordpress > Dashboard > Settings > Discussion > Default Avatar > "Letters (Generated)"