The artwork cacheing mechanism caches an image object that the retrieval code considers to be an unusable entry. In this case, it needs to note the audio file from which the image was derived and which can be used for mtime checks.

Additionaly, Slim::Schema:Track::coverArt is often used as a boolean. This is inefficient as it results in reading and examining the image, from image file or tag in an audio file. Better to use coverArtExists, which needs to be enhanced to know if knowledge of cover-art existance has been cached.

The attached patch deals with these issues.