That is because Photoshop broke your image preview. I'd be interested to know what version because this is a known issue (by me anyway) with Photoshop 5. My first suggestion is remove Photoshop from associating all file types except those that are only known by Photoshop. Then go to the image extensions in the registry. For example you are having an issue with .jpg. So go to Start | Run | regedit | OK. Then have the structure look like this:
HKEY_CLASSES_ROOT\.jpg\ShellEx
(default) value not set
HKEY_CLASSES_ROOT\.jpg\ShellEx\{BB2E617C-0920-11d1-9A0B-00C04FC2D6C1}
(default) {7376D660-C583-11d0-A3A5-00C04FD706EC}
This will get your image preview back. But the loss of the file association is another issue and Photoshop may remove this ShellEx key again.
You should have this key structure for these file types:
..jpg; .jpeg; .jpe; .gif; .bmp; .txt; .art; .htm; .html; .mht