Unlike JPEG, PNG is a lossless image format, which is why it reproduces pictures perfectly, but makes many of them — mostly photographs — much larger than what you’d get with lossy codecs. PNG is also the only way to get pictures with transparency (alpha channel) display in all browsers, so if we need images with alpha channel, we have to use large PNGs. However, there’s a little trick we can use to make them smaller: by applying lossy preprocessing.
MozJPEG is a project from Mozilla aimed to improve JPEG compression while keeping full backwards compatibility with current decoders. Recently they released version 3 of the library. Previous version reduced files by 5% on average compared to jpeg-turbo, the JPG compression library it is based on. The new version contains more improvements, such as reducing compression artifacts for text on white background, and better quality for high-resolution images.