This brings to mind an interesting point: to what extent are just the covers of an issue "canon?" Did the events on a cover "really happen?" How should they be treated?
If Superman has an eagle on a cover, does that mean he really does have an eagle somewhere?
This is particularly problematic because covers sometimes show events that don't actually HAPPEN in a comic! For instance, the cover to GIANT SIZED AVENGERS #3 has Thor, Iron Man and Vision saying something like "One of us will die in Limbo! But who? WHO?" Nevermind that they were teleported seperately and never actually MET each other until that question was answered, and also...how would they know one of them was going to die, anyway?
It may be that whenever a character remembers something, they remember it in terms of that one particular image that makes it to the cover...thus, covers are flashbacks to a very reminscent scene). You know, sort of like how lots of Baby Boomers only remember the sixties through Bob Dylan song lyrics.

Alternatively, it may be that especially memorable or splashy cover images exist in the photography of a world. For instance, in the Kurt Busiek MARVELS, tons of famous Marvel covers and panels existed as photographs taken by Phil Sheldon.
It is also possible that certain particularly memorable images that are covers in our real world, in the DC and Marvel Universes may exist as posters on college dorms, just like memorable photographs in our real world are subjected to this: the LIFE cover of a sailor kissing some random woman, or Mohammed Ali knocking that one guy down!
Another explanation is that in the "real" DC or Marvel universe, comics covers are the covers of liscenced comics BASED on real-life superheroes. Maybe that's one way Phil Sheldon made a few bucks on the side: letting his photographs be used to make covers!