In the case that you're no longer able to open your image files with "Windows Picture and Fax Viewer". Here's some small tips:
Valid for all OS. System Restore will solve your problem if it is recent. To do this, click on "Start, Accessories, System Tools, System Restore". Choose a restore point prior to the onset of the problem. This will be reset to the date of the Windows Restore Point.
Your data will not be affected
Disable Spybot resident
Try the right click:
Right click, open with, default program, and look for "Image Preview". Remember to check the "Always use this program."
It is important to distinguish the "Windows Photo Gallery" and "Preview image" feature.
Right click on the image
Open With -> Choose Default Program
Go to C: \ Program Files \ Windows Photo Gallery \
Click the bottom right of the dropdown and choose "All files" (instead of applications).
Find the file "PhotoViewer.dll" and double click on it.
Click OK and repeat the operation for the other image format.