I have searched many discussions, and can only find repeatedly the same advice, which doesn't work (that is, in Folder Options, ensure that 'Always show icons, never thumbnails' is not checked.
I also have used tried a few other things, including an program called TICR (to remove and remake thumbnails).

The problem is that only some icons show as thumbnails and others don't, and there seems no consistency as to which do and which don't. See example below (all are jpg files). This is really frustrating. I am not a computer expert, just a frustrated user! Please, if you are going to tell me to check my Folder Options, don't do so, as I have followed all the advice I can find, including that one. Help. Anybody?

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