I finally had a time to look at it:
Kodi seems to consume an insanely amount of (graphics) memory when browsing pictures, especially in wall mode with 4x3 thumbnails on screen.
RPi4 showed about 1GB memory usage and RPi3B+ with 256MB CMA quickly crashed because it ran out of memory.
I'm not exactly sure why kodi is doing that, maximum texture size on RPi0-3 is 2048x2048 (i.e. roughly 8MB at full HD, max 16MB for 2048x2048 at 32bit) and it looks a bit like it may be not loading the scaled-down thumbnails but allocating full textures for every picture.
Anyways, bumping CMA to 512MB looks like a good workaround for now until this is solved in kodi.
Another workaround would be to use list mode in pictures, that only shows one single picture and seems to use a lot less memory (check CmaFree in /proc/meminfo on RPi2/3 - on RPi4 graphics memory is allocated from heap so check with top there).
so long,
Hias