brew install imagemagick
The ImageMagick commands that modify images are mogrify and convert.
By default, the mogrify command overwrites the existing image with the modified image unless you specify an output folder into which modified image will be saved.
The convert command saves a new modified image and leaves the original unchanged so it is safer to use.
mogrify -path Newimage/ -border 1x1 -bordercolor "#000000" image.png
or
convert -border 1x1 -bordercolor "#000000" image.png Newimage/new-image.png
To reduce an image’s width to 790 pixels while maintaining the aspect ratio, and to avoid increasing the size of the image if it is already smaller than 790 pixels wide, use either of the commands:
mogrify -path Newimage/ -resize "790>" image.png
or
convert -resize "790>" image.png Newimage/image.png
convert image.png Newimage/image.jpg
The default JPEG qualilty is 92% but you may set it to any value from 0% to 100%.
For example, to convert all the PNG images in a directory to JPEG with 70% quality, enter the command:
convert -quality 70 image.png Newimage/image.jpg
convert -resize "790>" -border 1x1 -bordercolor "#000000" -quality 70 image.png converted.jpg
You can also use wildcards to batch process (but not change format or rename):
mogrify -path Newimage/ -resize "790>" -border 1x1 -bordercolor "#000000" *.*
If you want to change the format or rename a file in a batch, then you need the convert command, but that doesn’t handle wildcards so you need to do something like:
mkdir Newimage
for i in *.png; do convert "$i" "Newimage/${i%.*}.jpg"; done
To then combine those transformations:
for i in *.png; do convert -resize "790>" -border 1x1 -bordercolor "#000000" -quality 70 "$i" "Newimage/${i%.*}.jpg"; done