>>280301368
If you trust the hacker known as 4chan, here's a quick script to do it with the one ImageMagick file it needs to run bundled.
Just drop the png files you want to convert into the folder and double click the batch file. No idea if there are parameters that would compress it better, this was basically a zero effort thing.
https://mega.nz/#!VsEQEQaJ!0ZuDOwVS4aAHoj05tvpcpmXwn5szOgNoSka45Qbk4wU