[pkscout]

I went ahead and installed the Leia Estuary mod, and indeed was able to set the slideshow to play at proper aspect ratio and start the instant I start playing music. One remaining issue is that the colors still look not good (too low contrast - washed out), even after setting the Background diffusion to None. Strangely, the looks don't change if I switch to diffusion of 100%. I checked, and the same photos show nicely when viewed as Pictures on Kodi.  Am I doing something wrong here?

I don't think you're doing anything wrong.  The diffusion setting only applies to background images in the various media pages (TV Shows, Movies, etc), not the artist images on the music visualization page.  The only thing I can think of is to make sure you don't have the time progress bar (or the album art) on the screen.  That does include an overlay to mute the colors a bit so that text is readable.  On a keyboard at least the 'i' key toggles that overlay on and off.  Maybe post a picture?

Here's an example of what mine looks like with the info screen up (note the artist bio is optional on this screen).


And here's one with that info screen gone: