A very important part of listening to music as an experience is the visual media that is usually given along side it.
This goes beyond just the album cover. For example, CD releases of game soundtracks came with booklets that would include key art, character renders, story and world explanations, composer/developer commentary and interviews, sheet music, credits, and potentially even more album specific artwork. In the context of digital backups or releases, even more images can be included such as scans of the actual disc art itself or false disc artwork for the sake of novelty, screenshots, fanart, ect. These are all done by skilled artists in the same way that the music itself is with the intention for the listener to digest it simultaneously with whats being listened to.
In the same way that WACUP will use and read "cover.imgformat" as the default album art within the directory of an album, perhaps WACUP could read from a subfolder titled "images" in the same directory that would take the image contents of that folder and use it for the slideshow mode. Settings could include a shuffled display mode, an editable timer for how long images are displayed before switching to the next, or for the images to switch with playback of a new song in the playlist.