That's "cover" as in "album cover", not "cover tune". This is a feature on the cover of Over-Nite Sensation, with Dave McMacken, the artist who painted it.

(And so we're clear: even though it's just two dudes discussing an album cover -- still NSFW here.)

I told the story of when my dad bought his first three Zappa CD's and let me have two of them. He never did get much into CD's as a format, and this is part of why -- he showed me this album cover and said "Look at this tiny little thing. When you buy it on vinyl, it's this big; used to be when you bought an album, you got art."