Here is a plugin for Gimp which gives you a “Save for Web” option in GIMP. This plugin gives you the option of preparing your images for the web by optimizing it, adding additional compression, stripping EXIF information from the images to reduce it’s size. You can preview the resultant image before you save your final image.

You can get the deb files for this plugin over at GetDeb.net:
http://www.getdeb.net/app/GIMP+%22Save+for+Web%22+plugin

Just download the deb file for your version of Ubuntu and double click the deb file once downloaded to start the Package Installer.