Interesting default program,
Is that your choice of a default photo editor/ viewer - paint ?
If you use the preview pane do they display a photo preview,
My first thought is to change your default photo viewer/ editor, to wlpg or even windows photo viewer.
I have to say when people change the setting to always show thumbnails and never icons, they often forget to click on the top button apply to folders, and okay.
Only click on apply and okay is not enough, for the setting to be applied to all folders.
On the same view tab section in folder options,
Check the box to show preview handlers in the preview pane as well, apply to folders and apply and okay.
I have to say this is very strange,
Also I would suggest,
Click the start button, and type in the little explorer box, disk cleanup and hit the enter key or click on the suggestion link on the top,
Once it completes,
Make sure the temporary files boxes are checked and the thumbnails box is checked,
Then click on okay, and confirm yes permanently delete these files.
Windows will rebuild your thumbnail cache in windows explorer.