There are various images using in this library's distribution for testing. They are:

input.tif and it's variants:
Copyright (c) 2000 Andrew Cagney and release under the terms of the GPL. This image is one of a set of tutorial pages presented at AUUG2k, the 2000 annual conference of the Australian Unix Users Group (http://www.auug.org.au).

gnu_box.jpg:
By Stefan Kamphausen from http://www.gnu.org/brave-gnu-world/rungnu/rungnu-sk.html

gnu-head.*:
Author unknown, but heavily used on http://www.gnu.org

brave-gnu.png:
By Pierre Libotte from http://www.gnu.org/brave-gnu-world/rungnu/rungnu-pl.html