It usually happens if you removed a program that was set as the default app to open the file. So say you used photoshop to open all .jpg files and you uninstalled photoshop, or you changed it from the default "Windows Picture and Fax Viewer", it would change.

Try to open one, and when the "Open With" dialog comes up, choose the "Windows Picture and Fax Viewer" and the set it as default, or whatever program you want to open it with.