I have been looking into this and am planning to implement it such that you can specify additional folders to search over and above the current folder for the music file.
A folder can be specified as a relative (or absolute) path to the music file and can include tags in the construction of the folder path eg.
The order of precedence for the primary picture would be first matching artwork in the current file folder and then the additional folders would be searched. All matching artwork files would be associated with the music file.
When additional folders are specified, you wont be able to set the include sub-folders ie. you can either set "include sub-folders" or specify additional search folders but not both.
Does anyone have any comments on that approach?