What are some alternatives?
TinyPNG - Make your website faster and save bandwidth. TinyPNG optimizes your PNG images by 50-80% while preserving full transparency!
ImageOptim - Faster web pages and apps.
iLoveIMG - iLoveIMG is one of most powerful solution that comes with all the major tool you cloud want to edit images in bulk.
pngquant - command-line utility and library for lossy compression of PNG images
JPEGmini - JPEGmini - The Photo Optimization Tool Trusted by Tens of Thousands Image Perfectionists
Photo Size Optimizer - Photo Size Optimizer for Mac is a powerful photo resizer and optimizer to optimize photos by 80% ~ 20%.