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.
I noticed that when you're on a page of a band with a background picture, it takes the background from that band when you open the play queue. Could that be what happened?
I doubt it because I have absolutely no idea who those eight guys are - they aren't a band I listen to. The image for J bands is some hair rocker, and for D bands it's some (presumably) K-Pop act, none of whom are in my library.
Not that this helps you, but I think it's an image of T-Square. So probably Jellyfin treating your T folder as one whole artist.
This is what I suspected: if OP has a folder structure with folders for each letter AND all artists from the same letter share the same image, then quite probably Jellyfin is taking the parent folder for the letter as artist, looking for the closest artist name it can find and using that image for all the content of the folder.
Good find!