This is the result of the contrib/devtools/optimize-pngs.py script.
This saves the grand total of 14819 bytes.
My understanding is that the script doesn't impact the image:
it will optimize the png files, remove various color profiles, remove ancillary chunks (alla) and text chunks (text).
Optimization is done by using the filter with the highest compression method by brute-forcing all the filters. Because PNG is lossless, this doesn't impact the image.
Tested on current core master, the HD images are not optimized, I guess they forgot to run the script.