Replacing album artwork not working as expected

[rstemmer]

When trying to replace an album artwork via Web-Frontend, at least the cached down-scaled artworks get not updated.

Reproduce:

1. Have an invalid artwork that shall be replaced
2. Upload a new album artwork that should replace the old one

Expected: New artwork appears
What actually happens: The old artwork remains

[rstemmer]

Seems to be an cache issue of the browser. A browser that not has shown the old thumbnais used the correct version.
So they have been generated.

This still needs some investigation and code review. The whole code is quiet old and has not been developed with direct access to the artwork management in mind. Originally the artwork management was part of adding and updating whole albums.