There is an artistsearch text file in the "programfiles" > JRiver > mediacenter > data > default resources folder (structure depends on your set-up.
Create a "Custom Resources" Folder to store a copy of the artistsearch file in, because each time Media Center updates it will revert back to the original and you'll lose your work.
There you can add or edit what the program looks for to get rid of some of the strange pictures that crop up. Use any text editor (ie Notepad) and save as a text file only.
Example
The Cars
The Cars 80's rock band
Beethoven
Beethoven Classical Composer
The second line repeats the artist and adds a descriptive term to narrow Media Center's focus.
You can use ~music and other descriptors.
When you're done, save a copy in both directories so you won't lose your work when Media Center updates.
It's still not perfect, but it is better. You'll see some examples already in the file.
I recommend you edit in alphabetical order so you can easily find and re-edit artists you look for.
It's a little trial and error.
Hope this helps. There is a previous instruction that may be more clear than mine if you can find it.