What are some alternatives?
When comparing FileOptimizer and XnConvert, 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
Caesium Image Compressor - Compress your pictures up to 90% without visible quality loss.
ImageMagick - ImageMagick is a software suite to create, edit, and compose bitmap images.
pngquant - command-line utility and library for lossy compression of PNG images
Phatch - Phatch = Photo & Batch!