Tip: The mogrify command is part of the ImageMagick software suite.
Super tip: You can create an AWS Lambda layer with a static build of mogrify and perform these actions from within Lambda.
Resize all png files to a maximum width of 150 (or 600) and proportionately scale the height
mogrify -resize 150 \*.png
mogrify -resize 600 \*.png
Resample and set quality to 85%
mogrify -resize 1000 test.jpg
mogrify -quality 85 test.jpg
Rotate a PNG 90 degrees counterclockwise
mogrify -rotate "-90" test.png
Change all GIF files to JPG files
mogrify -format jpeg \*.gif