Explorer Preview pane image will not resize

Thanks once again. I hadn't considered my user profile, and was so hoping this might be it, but I created a brand new user and still the same behavior. I'll take more of any suggestions you might have, tho.

I looked at both shellexview and shellmenuview. I used the first to disable anything that had to do with images, but none seemed to affect the preview pane. I saw that shellmenuview related to context menus, and disabling took the related menus away as expected, but this also did not seem to affect the preview pane behavior at all.

Also, yes, the preview handlers in preview pane is checked. I've tried unchecking this as well, and I can't tell what that's supposed to change. I can still preview and it still shows the handler list even when this is unchecked.

Thanks so much for your help on this!

These are things I did also think of, so yes:
1) PDF viewing is perfect; they resize with the pane. Also MS Office docs (Word, Excel) expand correctly, so it does seem like it is just images.
2) Default was Windows Photo Viewer. I tried setting this to Photoshop, and also Paint. But the problem still persists. I even downloaded InfranView, as this was suggested as a better image viewer in general, but it also did not work, and I uninstalled that. Currently it is set back to Photo Viewer.

I went ahead and ran SFC /SCANNOW. It repaired a few corrupted files, but no change. Windows version is 6.1.7601

On my computers the size of the Preview Pane determines the size of an image. On the column headings with Name, Date, Size, etc., moving the mouse just a bit to the left of the image should get a double-point arrow and click to move that to the left makes the image bigger. It's a very faint line on most displays. In this image it's just to the right of Tags:
Attachment 192969
A lot depends upon the Display Resolution and maybe the icon/text size, I do better with 125% on 20" and larger panels.

Yes, I'm also at 125% on a 27" monitor at 3849x2160 - I've tried making that 100%, but no change, and have tried making the resolution 1920x1080, no change. My other computers are at high res as well, but do not behave this way - they behave just as you portray in your last post with image. I think I will in fact do the in-place repair install. I've been thinking this way for some time because of this. But, being somewhat of a nerd I also wanted to solve it, just to add to my arsenal of fix-its! If I do go the route of reinstall, I'll post the results.