YOGA Image Optimizer is a free tool to convert and optimize images. It currently supports JPEG, PNG and WebP as output formats and much more image formats are supported as input.

With YOGA, you will be able to save about 30 % of space on JPEGs and 20 % on PNGs. On WebP images, you will only save few percents... but converting a JPEG to a lossy WebP can reduce image size to a half and converting a PNG to a lossy WebP can save you 35 % on average.

Please note that image optimization can be slow and may require a lot of RAM, especially for JPEGs. For example, the JPEG encoder needs 300 MB of RAM and 3 min per Mpix (PNG and WebP encoders are less resource hungry).