GTK+ is a potential source for some of the image libraries that Emacs
requires. GTK+ is installed along with other ports of GUI software,
such as the GIMP image editor, and Pidgin instant messenger client.
If GTK+ is installed when you run addpm, Emacs will use the
image libraries that it provides, even if they are not on the
PATH. GTK+ ships with JPEG, PNG and TIFF support.