On some of the cars I've made I've used a web app called TinyPNG (
https://tinypng.com). It reduces the filesize while preserving the alpha channel (transparency). But I also discovered a command line version called pngquant (
http://pngquant.org) that apparently uses the same algorithm that TinyPNG uses. There is some loss in image quality, but I've personally not found it very noticable. Anyway, I though it might be a handy tool for SD.