I'd like to suggest to have a crop option (left / right + top / bottom) as well, as the adjusting to fit the cover image for the webgui?
I know, its hard to find the "best looking" solution, as coverart, which has no aspect ratio of 1:1 (height:width), is difficult to fit in a 1:1 box, but
in most cases, cropping looks "better" (as in pleasant for the eyes and not irritating to the memoized image of a human) then stretching.
So it could be nicer to view a coverart which is cropped at both sides (left/right or top/bottom).
The attached image is surely not the best, but a good example, as it covers both, a good and a bad example of a coverart:
image ratio not 1:1, but the content/matter/recordlabel is 1:1
label is not centric (right distance to the border is bigger then the left distance)
easy to crop (at least for humans and advanced software/scripts)