I had put the wrong title. The images are transparent and I want them to appear with a list layout on a white background (via the back option). However, the background is black when I open the panel. It works OK with cards layout, though.
I’m using the default thumb driver in Kirby 3.2.3 which, looking at the source, appears to be gd.
It might be a problem with the driver (I’ve seen issues here with transparent PNGs getting a black background here), but the strange thing is that in your case it only seems to affect the small thumbs, not the bigger ones (unless maybe the file is not resized for card style).