Silence isn’t the only way to leave style guidelines loose. We could also publish a style guideline with loose guidelines. We can have guidelines that say, some is better than none, and good is better than some. And, wrong is bad (don’t give one Release the cover art from another Release, even if it’s in the same Release Group).
From the discussion I’m seeing the outlines of a principle about what “better” is. High fidelity is good. Enough pixels to provide fidelity is good. Fewer pixels, or too many pixels, is less good than enough pixels. But contributing an image with lots of pixels in high fidelity is better than not contributing because it’s too much effort to reduce the image to fewer pixels.
It would be helpful, I think, to give guidance about whether it’s OK to use the pristine digital artwork from a digital-only Release as the cover art for the corresponding CD Release. This is essentially the “wrong is bad” case above. But maybe the pristine digital artwork matches the appearance of the CD cover, in which case maybe it’s not wrong.
Even the two paragraphs which @reosarevok quotes above would satisfy my desire for “some minimal Cover Art style guidelines”.