The photo viewer is supposed to autosave the last modification by default. I just rotated a few images on my own machine- after I click on the top right X to close the viwer, I see a momentary "saving" notice before the window closes. The images are still there on the hdd and on reopening, they show up as last rotated.
I suggest you run a sfc scan 3-4 times.
SFC /SCANNOW Command - System File Checker