(Redirected) Photo Icons

Hi there I am using Windows 7 on a Dell Desktop and for some reason my photo icons have all turned into the same picture which is a pine tree on yellow hills.  I have thousands of photos and used to have previews of those photos as icons but this has changed recently and I don't know how. 

I have tried right clicking and changing the properties and also opening with different programs but nothing works. 

Is there anything I can do to get all the previews back. I have also changed the icon sizes etc but it doesn't make a difference.

This makes it very difficult to find photos.