are you referring to the picture in the track info panel?
if there is no artwork embedded in the downloaded file, then MB falls back to the subscription picture which is stored in:
AppData\InternalCache\Subscriptions
(help/support/Open AppData folder)
If there picture is no picture named after the subscription or its corrupt, MB falls back to a default image.
You could try deleting the picture and MB will automatically redownload it on the next subscription refresh