Yeah, just look through progarchives, there's some pretty awesome prog covers out there.
I love Roger Dean and Hipgnosis's covers. Mark Wilkinson has some good ones too.
Gong, Hawkwind and Ozric Tentacles had some great illustrated covers to name a few, as well as some of the more obscure prog greats.
Though admittingly there's some really bad prog album art too. Camel's debut album and Gentle Giant's Acquiring the Taste come to mind. And of course the legendary Love Beach from ELP.
And Birth Control, I can't think of anything more mind numbingly awful than their album covers.