On both the Desktop and in Dolphin, some files do preview/thumbnail, while others do not. They're not different file-types:
Some jpgs thumbnail, while other jpgs in the exact same directory don't.
Some pngs thumbnail, while other pngs in the exact same directory don't.
There isn't a big difference in file size, so that shouldn't be the issue.

Do the image files have the correct filename extension? If unsure, use Okteta to check the file contents: PNG starts with 'PNG', while JPEG has 'JFIF' near the beginning.

kde-cfeck wrote:Do the image files have the correct filename extension? If unsure, use Okteta to check the file contents: PNG starts with 'PNG', while JPEG has 'JFIF' near the beginning.

Good thought, but they are overwhelming correct. There might be a few with the wrong extension, out of probably a hundred that are not displaying properly--all over the drive.