You'll need to install ImageMagick
- On Windows - https://www.imagemagick.org/script/download.php#windows
- On macOS - https://www.imagemagick.org/script/download.php#macosx
Here's a basic tutorial on using Windows command line - https://www.computerhope.com/issues/chusedos.htm
And on macOS terminal - https://macpaw.com/how-to/use-terminal-on-mac
The command
mogrify -format jpg -resize 800x800\> *
will change the format to jpg and shrink images with dimensions larger than the corresponding width and/or height arguments. The *
will apply the command to all files in the current directory. You could limit this to all files with a certain extension by using something such as *.JPG
in the command instead. See https://www.imagemagick.org/script/command-line-options.php#resize