As far as I can tell, we have no (zero) style guidelines for Cover Art.  The closest we have is documentation on the Cover Art feature, and How to Add Cover Art instructions.

Considering how important Cover Art is, and how it relates to distinguishing whether two physical products are part of the same Release or different Releases (artwork differences mean different Release), there should be Style guidelines.

I propose that we get a rough consensus about some minimal Cover Art style guidelines, write a first revision of the page, and go from there.  There should be links to this page from the Cover Art documentation and the How to Add Cover Art instructions.