This is awesome!
But I believe this setting shouldn't be file dependant and after choosing background image it should be automatically copied somewhere in MB's AppData, that way removing or renaming image file wouldn't affect MB as it does now.
Also there is a minor bug, if artist biography is double clicked it loses "transparency" that allows custom image under skin to be seen and it turns into skin's solid colour. Restarting MB revert it back to custom background image in that part of player. Same is after double clicking lyrics, but when editing lyrics is done it reverts custom image back to that part of player without need to restart MB.