This is going to get deep fast. File associations matter as the app the files are associated with provide the image decoding.
https://www.sevenforums.com/tutorials/19449-default-file-type-associations-restore.html has almost all the default associations for most file types.
You would have to run the jpg and jpeg reg files and reboot to see if that's it.
I have a problem with Large and Extra Large thumbnails of photos not showing. Medium thumbnails show fine. But when I switch to Large or Extra Large, they flash as thumbnails, then revert to the mountain scene.
I've done the two solutions mentioned elsewhere - I've unchecked the box in Control panel, Folder Options "Always show icons, never thumbnails" and I've deleted my Thumbnail cache several times using Disk Cleanup. But the problem remains.
I actually think the problem started when I ran Windows Cleanup, but I'm not sure.
What else can I do?