CLI Fu

Image Magick

Convert, noclobber:

ls webp/*webp | sed 's/webp//g' | awk '{print "[ -f png"$1"png ] || convert webp"$1"webp png"$1"png"}'

Convert, noclobber, thumbnail:

ls png/*png | sed 's/png//g' | awk '{print "[ -f jpg"$1"jpg ] || convert -thumbnail 256x256 png"$1"png jpg"$1"jpg"}'