Unless otherwise noted, the *.gz files were generated by the gzip command line tool and the *.deflate and *.zlib versions were then generated by script/extract-deflate-offsets.go. Similarly, the *.giflzw files were generated by script/extract-giflzw.go and the *.palette and *.indexes files were generated by script/extract-palette-indexes.go
The *.jpeg files are usually the canonical versions of the test/data images, and other versions (*.bmp, *.gif, *.png, *.tiff) were generated by ImageMagick's convert command line tool. The *.webp versions were generated by the cwebp command line tool.
The artificial directory holds artificially generated test data, often to explicitly test corner cases of various file formats. The files there usually come in families whose names have a common prefix, such as three files “foo.bar”, “foo.bar.qux”, “foo.bar.commentary.txt” all prefixed by “foo.bar”. The file whose name is that prefix is usually the canonical (typically hand-crafted) file, and the other files are then derived from that. Outside of the artificial directory, the other files in this directory are typically real world examples of various file formats, or deriviations of them.
animated-red-blue.gif is an original animation by Nigel Tao email@example.com.
bricks-* are various encodings of an original photo by Nigel Tao firstname.lastname@example.org.
crude-flag.* is an original animation by Nigel Tao email@example.com. See the
gifplayer-muybridge.gif is an original animation by Nigel Tao firstname.lastname@example.org.
harvesters.* are various encodings of a photo of “The Harvesters” by Pieter Bruegel the Elder, held by the Metropolitan Museum of Art.
http://www.metmuseum.org/art/collection/search/435809 lists that image as in the public domain.
hat.* are various encodings of a photo of “Self-Portrait with a Straw Hat (obverse: The Potato Peeler)” by Vincent van Gogh, held by the Metropolitan Museum of art.
http://www.metmuseum.org/art/collection/search/436532 lists that image as in the public domain.
hibiscus.regular.* are various encodings of a photo of “Hibiscus and Parrots” by Louis Comfort Tiffany, held by the Metropolitan Museum of Art.
http://www.metmuseum.org/art/collection/search/13503 lists that image as in the public domain.
hibiscus.primitive.* is the result of running the hibiscus.regular.png image through
github.com/fogleman/primitive and other programs:
primitive -i hibiscus.regular.png -o hibiscus.primitive.svg -s 442 -n 32
inkscape -z -e hibiscus.primitive.png hibiscus.primitive.svg
hippopotamus.* are various encodings of a cropping of a photo of “Hippopotamus (William)”, held by the Metropolitan Museum of Art.
http://www.metmuseum.org/art/collection/search/544227 lists that image as in the public domain.
http://www.metmuseum.org/about-the-met/policies-and-documents/image-resources says that “You are welcome to use images of artworks in The Met collection that the Museum believes to be in the public domain, or those to which the Museum waives any copyright it might have, for any purpose, including commercial and noncommercial use, free of charge and without requiring permission from the Museum.”
midsummer.txt is an excerpt of Shakespeare‘s "A Midsummer Night’s Dream", copied from
muybridge.gif is derived from
https://en.wikipedia.org/wiki/File:Muybridge_race_horse_animated.gif which is in the public domain.
pi.txt contains the digits of pi.
pjw-thumbnail.* are various encodings of an image derived from an iconic, original photo of Peter J. Weinberger by Rob Pike email@example.com.
romeo.txt is an excerpt of Shakespeare's “Romeo and Juliet”, copied from
romeo.txt.fixed-huff.deflate was derived from romeo.txt by a custom program to use fixed (not dynamic) Huffman tables for the deflate encoding.