I went back to my album-cover images (all 600 x 600, named albumart.jpg ), which I had collect off the Internet. There all appear flawlessly on Linux or Android, in various music appllcations (or image viewers).
I opened each with GIMP v2.8 (under Ubuntu 14.04 Linux). One image had an embedded colour profile of ‘Adobe RGB (1998)’, and I was prompted to convert this to sRGB. I flattened each image, then exported at (default setting of) 80% quality. Then used my standard script (which uses ImageMagick v6.7) to resample all images to 192 x 192 , output as filename folder.jpg.
When transfered to the Clip Sport, all four images now behave identically. Now images 3 and 4 are also 52 x 52 images.
That was not what I wanted, but I still have my earlier images so will continue …