I’m experiencing the same bug.
After I finally got Daemon Gnone, I noticed I hadn’t downloaded all assets after the update. I downloaded the rest, restarted the game, and Daemon Gnome still didn’t have a picture. Restarted a few more times without any changes.
The last time I had a troop picture problem, I had to uninstall the game completely and reinstall it. I’d like to avoid doing that if possible.
EDIT: Ah, I thought I had read about this earlier. It’s a known issue and they were looking into it a few days ago.