An outboard brain post.

To resize all images in a set of subdirectories:

find ../ -name "*.jpg" -exec mogrify -resize 192x192 {} \;

To check which files this will impact, you can run it non-destructively by placing echo in front of the mogrify command:

find ../ -name "*.jpg" -exec mogrify echo -resize 192x192 {} \;