Optimising images for the web is a tricky business. You have to get the right balance between filesize and picture quality. It is an essential step though. Look at any webpage, and you will see that most of its load time comes from images. Your website will be needlessly slow if you don't drop the sizes of these images.
If you give importance to the three key areas where bytes can be shaved off your graphics: bit depth (number of colours), resolution, and dimension the pages can be loaded faster.....