It’s challenging to automatically choose the best JPEG quality and quantize PNG images. Moreover, many tools give inconsistent results, e.g. while zopflipng gives almost optimal deflate compression, it does only basic pre-compression optimizations (from choosing PNG type to palette sorting and dirty transparency). These reductions can result in huge savings. Color management is also important. I’m working on a tool Optimage (getoptimage.com) to actually solve most of these problems and provide visually lossless optimizations.