Explorer is looking for the shell command HKCR\SystemFileAssociations\image, which should be "C:\Program Files\GIMP 2\bin\gimp-2.8.exe" "%1"
[HKEY_CLASSES_ROOT\SystemFileAssociations\image\shell\Edit with GIMP]
@=”Edit with GIMP”[HKEY_CLASSES_ROOT\SystemFileAssociations\image\shell\Edit with GIMP\command]
@=”\”C:\\Program Files\\GIMP 2\\bin\\gimp-2.8.exe\” \”%1\””