The cover for Memory is just a cover. I am not passionately opposed to it (as I am to the cover for Limit of Vision). It's true that it is dark, and hard to make out any details or meaning, but maybe that is not a bad thing.
The cover is an attempt to show the structure of the story world, which is described as a great ring -- however it has managed to get it completely wrong, since the habitable part of this ring is on the outside, not the inside, meaning one would never see this great span rising into the sky. (This is not Ringworld.)
In fact, the actual shape of the world has very little impact on the story. It's just a prop. In my opinion, a ghostly cover that plays with the haunting fogs of "silver" would have offered a lot more dramatic potential. Ah well.