This worked for me:

1. Put the images I wanted resized into a folder.


2. Opened the folder into a terminal window.


3. Installed GraphicsMagick (an active fork of ImageMagick) using sudo apt install graphicsmagick-imagemagick-compat


4. gm mogrify -geometry 600^x600^ *.png

The gm command shown above worked very quickly to resize all the png images in the directory. The ^ symbol ensures that the image aspect ratio is maintained when the image is resized, but the resulting width or height are treated as minimum values rather than maximum values.