#1 The right and bottom corners look different from top and left for example. (You might need to zoom in to see what I mean)
#2 BTW, I have no idea how feasible it is but... this plugin might open for even more cool stuffs. For example custom margins, shadows, outlines, ...
#1 A difficult one to spot, but it should be fixed now if you redownload the plugin - all four corners are now even.
I have also added two hotkeys: (i) Artwork: Rebuild Cache and (ii) Artwork: Round Corners.
While (ii) now alleviates the need for a restart, (i) still requires it because MB only rebuilds the cache folder on startup - all the hotkey will do is delete the cache folder for you.
#2 Yeah, that's what I'd thought to myself too.
Using the same technique, I was able to convert the artwork to grayscale which paved a way for a full-blown dark skin (albeit one that won't be popular amongst folks).
But yes, there's room for achieving some naughty aesthetics with this.
MusicBee's skinning options already support outlines. But yeah, things like glow effects, changing the perspective on the artwork have come to mind too.