Not to be too old fashioned, but... can't you just resize them yourself? I never add album art to anything without resizing it to what I want.
that's fine enough but doing it for 500+ albums is a bit crazy. i also don't enjoy altering the image itself to stretch to strange proportions, i like to keep the original untouched image
If it's optional, I don't care. But if it's supposed to be the default, then -100. Covers are not always square. I wouldn't want my rectangular covers elongated or flattened.
definitely optional, i don't think i ever recommend anything to be the default. you're thinking of a stretch fit (think wallpapers, there are tile/stretch/fill/center/span etc. options), fill is where it simply zooms in more till it fills the desired size