The usefulness extends to mobile platforms as well. While (AFAIK) the Android SDK optimises images to some minor extent as default when building an APK, I still like to run the images through various tools to get the best result.

On one of my previous apps, this resulted in a filesize decrease of approximately 300 kilobytes - effectively halving my apps size with no noticeable difference to the end user. Win!

There are 3 tools that I use extensively:

* jpegMini, for removing EXIF info from jpegs

* optiPNG, for removing extra pallettes from pngs

* gifsickle, to optimize gif's

It's amazing on how much junk bytes you can have in your images. I used them for everything, from my picture collection to my site porfolio.

ImageOptim and JPEGMini have both become parts of our pre-launch optimization, and they can easily save 50-70% of the image sizes.