RIOT has an easy to use interface to compare the original with the optimized image in real time and instantly see the final file size. Image Optimizer is lightweight, fast and easy-to-use, yet powerful for advanced users.

RIOT Radical Image Optimization Tool:

The main features:
      * open many kinds of images, including rare / scientific types
      * save and optimize JPEG, GIF and PNG with a simple, clean user interface
      * automatic optimization selects the most suitable format and parameters
      * works in a dual view: (original - optimized image) or single view (optimized image).
      Automatic preview of resulting image
      * compare features on the site (alternativelly display the original image over the optimized image to notice small pixel changes)
     * compress files to the desired file size threshold
      * fast processing (all is done in memory);
      * see instant results including resulting file size
     
 More Details click Here