PNGs will be made smaller by running them through two tools. The first one is Pngquant 2, a lossy PNG compressor. We set no extra options, their defaults are used. After that we run the image through a second one: Optipng. These options are used:
-i0: this will result in a non-interlaced, progressive scanned image
-o2: this set the optimization level to two (multiple IDAT compression trials)
SVGs will be minified by SVGO. SVGO's default configuration will be used, with the omission of the cleanupIDs plugin because that one is known to cause troubles when displaying multiple optimized SVGs on one page.