Do you get some error message when you try to open them by double-clicking ?
What happens if you click on the file once to select it, and then press the enter key ?. Does it open ?
Have you restarted the machine to see if it helps ?
I know you have not changed any settings, but, did you install some new software recently ?
Right-click on any image file, choose “Open with”->”Choose program”, and select the program you use to see image files from the list, then check the “Always use the selected program to open this kind of file”, and click “Ok”. Close the image file, and try to open it by double-clicking on it.