It's obvious that covers aren't narrative in a void, but there is an argument to be made that covers can be part of the narrative in some cases; certainly, illustrations can be part of the narrative of a novel or other prose story (we'd certainly be insane to think Dr. Twelfth primarily consists of the written text).
And while some covers are more like advertisements, featuring images of the characters' faces in an imaginary situation, there is the occasional cover which illustrates a specific event from the story, like any other illustration.
(Incidentally, T:NPOV and T:VS taken together should take care of your distaste for the Birth of a Renegade illustration's assertion that an early incarnation of the Master happened to look like Tremas. It's weird, but it's not impossible within a big strange universe like the DWU, and it seeming discontinuous with other stories proves nothing.)