What are some alternatives?
When comparing Caesium Image Compressor and Mass Image Compressor, you can also consider the following products
ImageOptim - Faster web pages and apps.
FastStone Photo Resizer - Convert and Rename images in batch mode Support JPEG, BMP, GIF, PNG, TIFF and JPEG2000
Squoosh - Compress and compare images with different codecs, right in your browser
XnConvert - XnConvert is an easy image converter for graphic files, photos and images available on Windows...
FileOptimizer - FileOptimizer is a lossless file size optimizer supporting AIR, APK, APNG APPX, BMP, CBZ, DOCX...
pngquant - command-line utility and library for lossy compression of PNG images