Because PNG works best for drawings and simple graphic representations, you can often reduce the size of PNG images to a fraction without making any noticeable difference in the appearance of the picture. Often it is easiest to do image work locally on a PC. Here are some free tools that will save you time.
Many drawings consist of only two or four colors. A standard PNG file has 256 colors. By reducing the number of colors to 2, 4, or 16 colors, the PNG file becomes extremely smaller. Without any visible change to the picture.
brew cask install imagealpha
brew install optipng
brew instal pngquant