Test images are available for most formats supported by the library.
Most of the images included in the test kit are also part of this
documentation (albeit in TIFF rather than GIF or JFIF).
The images are kept in a separate archive that should be located in
the same directory as this software.
The latest archive of test images is located at https://download.osgeo.org/libtiff/pics-3.8.0.tar.gz
There are two other good sources for TIFF test images: the contributed software contrib/dbs includes several programs that generate test images suitable for debugging, and the tiffcp program can be used to generate a variety of images with different storage characteristics.