PageSpeed Optimization Filters
The 'Optimize Images' optimization stage comprises three optimization passes. Enabling rewrite_images implies enabling
In every case, the image on the page is replaced by the optimized version only if the optimized version is actually smaller than the original.
pagespeed EnableFilters rewrite_images
Enabled by default via
pagespeed DisableFilters rewrite_images
Chance of decreasing image quality, depending on recompression settings.