There is a tweak for preventing the picture viewer from opening your pictures by typing "regsvr32 /u shimgvw.dll" at a command prompt.  This works, except if you use it, then if you set a folder to thumbnail display mode, your thumbnails will not work.

If you want to disable the picture viewer but would still like to have your thumbnails, then change this in the registry:

HKEY_CLASSES_ROOT\SystemFileAssociations\image\ShellEx\ContextMenuHandlers\ShellImagePreview\(Default)

It should be set to {e84fda7c-1d6a-45f6-b725-cb260c236066} by default... just erase its contents to disable the picture viewer.  Your thumbnails should still work.  If you did do "regsvr32 /u shimgvw.dll" at the command prompt, you can undo it by going to the command prompt and typing "regsvr32 shimgvw.dll"