Dave Newton explains how to use ImageMagick to quickly resize your images, while maintaining great visual quality and really tiny file sizes.

Source: Efficient Image Resizing With ImageMagick

This comment was the one I found very useful:

Nice article!

I also use IM to resize and sharpen images, but mostly for the better quality output compared to Photoshop

Here is my script:

for file in *.jpg; do convert $file -resize 1012x -unsharp 0x0.55+0.55+0.008 -quality 90% small-$file; done