GIMP Plugin Registry

Radical Image Optimization Tool (RIOT for short) is a free image optimizer that will let you to visually adjust compression parameters while keeping minimum filesize.

It uses with a side by side (dual view) or single view interface to compare the original with the optimized image in real time and instantly see the resulting file size.  read more »

This script is meant to cut an image is smaller pieces and save these images automatically based on the size and names of specific layers, images to be used in web layouts.

Other options I found are based on guidelines. This approach has 2 drawbacks for me :  read more »

Save for Web allows to find compromise between minimal file size and acceptable quality of image quickly. While adjusting various settings, you may explore how image quality and file size change. Options to reduce file size of an image include setting compression quality, number or colors, resizing, cropping, Exif information removal, etc.

If you want to send translations/contribute, please visit http://github.com/auris/gimp-save-for-web