All my music is in mp3 format as well, with metadata edited in Mp3Tag... I had album art in XBox Music on W8 and 8.1 just fine but nothing in Groove.
Edit: By the way, this comments section really kills my Lumia 1520. There's a two second delay for each letter or digit I enter, so by the time I'm done with a sentence, the text on the screen is several words late. And fixing spelling errors is a complete nightmare, as the cursor is all over the place! I managed to ride from one train station to the next in the time it took it to finish the last sentence after I stopped typing!!! Just before, it actually crashed the phone!