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.