I tried the same. First I checked, if the resized jpgs are the same as the cover.jpg. And yes they were the same, so Sooloos changed them after I changed the cover art.

Now I changed it to a wrong one and only the cover.jpg was changed, not the resized ones, even after restarting all components of the whole system. I keep an eye on the album and will check if the resized will change in future...

BvH

Control 10, MS600, Twinstore, Odeon 26S, Audiomat Recital