What are some alternatives?
ImageOptim - Faster web pages and apps.
XnView MP - XnView is a free software that allows you to view, resize and edit your images. It supports more than 500 different formats!
pngquant - command-line utility and library for lossy compression of PNG images
IrfanView - IrfanView ... one of the most popular viewers worldwide.
PNGGauntlet - -Combines PNGOUT, OptiPNG, and DeflOpt to create the smallest PNGs
XnView - XnView is a utility for viewing and converting graphic files.