Mozilla has done a study of image formats and concluded that WebP and JPEG XR are not a big-enough improvement over well-optimized JPEG. In the study only HEVC (H.265) was significantly better, but it's a patent-encumbered format, so it can't be used freely ( shhhh!)
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ked tak pozeram, co to dokaze s textom a komixovymi obrazkami, mozno by to vedelo priniest zvysenie kvality aj nasim obrazkom ukladanym v JPEGu