Test suite for GIF containing images using rarely used features

Question

The specifications (plural, since there are versions 87a and 89a) of the GIF graphics format are easy to find in the internet (for those who don\'t want to google:

Answer 1

I maintain a collection of test images for several file formats here:

https://github.com/drewnoakes/metadata-extractor-images

It contains those from the ImageTestSuite mentioned in Leo's answer, along with others from various contributors over the years.