Some imaging programs corrupt these entries.

You can check your registry settings for image file types:

Go to Start | Run and type regedit and look at this key:

HKEY_CLASSES_ROOT\.gif

On the right you should see

(Default) "giffile"
Content Type "image/gif"

for jpg it should be

HKEY_CLASSES_ROOT\.jpg

(Default) "jpegfile"
Content Type "image/jpeg"

same for \.jpeg

for bmp it should be

HKEY_CLASSES_ROOT\.bmp

(Default) "bmpfile"
Content Type "image/bmp"

Read the help file in regedit if you haven't used it before.