As far as git is concerned, the PNG file is just a bag of bits. It will record the changes in the IDAT chunks just as with changes in any other files. The images would appear identical on GitHub because they haven't changed visually.

pngcrush optimizes the file losslessly. Looks the same, smaller file size. Since the file does change, git will see a difference (doesn't care what it looks like). To a user, viewing the diff through this Github feature, it would look like nothing changed between commits.