What are some alternatives?
When comparing Mass Image Compressor and ImageOptim, you can also consider the following products
Caesium Image Compressor - Compress your pictures up to 90% without visible quality loss.
Transmit - Transmit is an FTP client for Mac OS X and Mac OS Classic (which is unsupported).
Squoosh - Compress and compare images with different codecs, right in your browser
CodeKit - CodeKit allows you to optimize the performance of your website by automatically and efficiently compiling a variety of popular languages.
FileOptimizer - FileOptimizer is a lossless file size optimizer supporting AIR, APK, APNG APPX, BMP, CBZ, DOCX...
TinyPNG - Make your website faster and save bandwidth. TinyPNG optimizes your PNG images by 50-80% while preserving full transparency!