The size and quality of images are now an important factor in the Search Engine Optimization. Images not only must be of the correct size, it must also be compressed. For PNG files, it is recommended to use PNGOUTWIN. This program can compress up to 20 files at one time during the trial of 30 days. Subsequently, you have to page about USD 15 to continue to use it.
As for JPG files, jpegtran is recommended. But it is a tough program to use because you have to go back to the basic, using DOS command. Thank God that I googled and found this solution which allow me to process the files in a folder instantly. You can find the method in this link http://www.darksiderg.com/forums/lofiversion/index.php/t56255.html.