Because that is not Canadian Metal four piece Tomb Mold. (They're pretty dorky looking too, it turns out, but that ain't them.)
I think I know why it's doing this: because my music library is organized under directories for each letter of the alphabet (/albums/T/Tomb Mold/...)
The same picture of eight Asian guys shows up for Talking Heads, too, for example. I'm just wondering if there's a way to specify a better placeholder image. I'm perfectly happy to go digging in the server's directories if that's what it takes.
Definitely your folder structure causing issues, if you go to the artist page within jellyfin, you can change the images via the edit images option. Failing that you can insert your on image into the relevant folder, iirc the backdrop images are named fanart.jpg.
When I Edit Images for the band, the Logo, Primary, and Backdrop images are all appropriate, so it's not using one of those (you'd think it would be Backdrop...) so I guess I'm digging for fanart.jpg.