[IAmEveryone]

GitHub doesn’t “optimize” your images, just as it doesn’t format your code. People would storm the barricades if Github changed their files.

[SahAssar]

They do change files that are embedded in readmes, just like most web platforms do. The original file is unchanged, but the version embedded in the readme preview on github.com is changed.

[angristan]

They just proxy it on the fly with camo: https://github.com/atmos/camo