I have a similar problem, but no solution. I have a Sony NW-ZX507 Media Player (Android device). I have two folders containing .JPG images on an SD card – one contains Artist Images, the other Album Covers. Both folders show up in F-stop, but the Album Cover folder displays blank black space where the image should be – the folder name displays correctly, the file count is correct and the grid is all there; image filenames and size are correctly displayed.
The Album Artist folder displays its content perfectly.