Imagemagick is the ideal tool to scale, convert or crop a lot of images. The last version of Imagemagick support construction with * to work with a lot of images. But if your images are scattered over a lot of folders and subfolder you need another construction:
The next command will convert al jpg to new jpg with 75% quality, the maxdepth options specifies how much subfolders it will do:
find . -maxdepth 1 -name '*.jpg' -print0 | xargs -0 -I {} convert -quality 75 {} \
/tmp/thumbs/{}
To compress and resize inplace:
NOTE! Mogrify is a dangerous command because it operates on the original files! Test your commands on COPIES of your files.
find . -maxdepth 1 -name '*.jpg' -print0 | xargs -0 mogrify -resize 800x600 -quality 75
To change file types:
find . -maxdepth 1 -name '*.jpg' -print0 | xargs -0 mogrify -format png
The above statement will leave the original JPG files alone, and write PNGs beside them.
Same from png -> jpg with scaling 50% applied:
find . -maxdepth 1 -name '*.png' -print0 | xargs -0 mogrify -quality 80 -scale 50% \ -format jpg
The above statement will leave the original PNG files alone, and write JPGs beside them.