Having just recently returned from holidays with a ton of photos to sort through and upload/email to friends and family I didn’t want to also spend hours uploading huge 10 megapixel photos at roughly 5MB each.
Mogrify is a powerful image manipulation tool which comes as part of the ImageMagick bundle. It allows you do a whole host of image enhancements and modifications including brightness, contrast and sharpness tweaks, converting to black and white, as well as converting image formats and even adding signatues, borders and waterstamps. Because mogrify is cli based it is easy to perform bulk image modifications of entire directories and can even be used in scripts to perform all of these enhancements at once.
Here is the command I used to resize my holiday snaps:
mogrify -resize 40% ~/Holiday_Photos/*.JPG
As an example I was able to reduce a 5MB @ 3872×2592 photo to under 1MB @ 1549×1049
If you would also like to add a signature to your photos you can do so with the following command:
mogrify -font /usr/share/fonts/truetype/msttcorefonts/arial.ttf -pointsize 42 -verbose -gravity SouthEast -draw 'fill black text 20,20 "www.homecomputerlab.com" fill white text 21,21 "www.homecomputerlab.com"' *.JPG