I was thinking about optimizing PNG on various sites with high traffic. So I tried Wikipedia: I right clicked from my browser on the Wikipedia logo (the one we can see on every pages), then saved it to my computer. I ran a PNG optimizer which combine many optimizers (ImageOptim) then I saw that it has reduced the size of the image by 5,9%. It's not huge, but I thought it could be a good new for Wikipedia (I think the logo is downloaded thousands times a day).
So I sent a mail to Wikipedia support team (OTRS info-fr) and Thibaud Payet encouraged me to open a ticket. That's why I open it now!
The optimized image is here: http://raph.site/frwiki-2x.png As you may know, PNG optimization is lossless, so it it's exactly the same quality. I asked on stackoverflow about CPU load client side for decoding optimized images, and it does not seems to have any impact (I also read that it may have a positive impact even client side!). more info here : https://stackoverflow.com/questions/45058742/png-optimization-and-environment-impact/45071942 and there https://stackoverflow.com/questions/28452429/does-gzip-compression-level-have-any-impact-on-decompression
I would be happy to optimize more image for you (also, I would be really proud too!).
Side note: it's the french logo, because I'm french, but I can try on some other logos!