Image optimization will reduce loading time even though some users have slow connections however more and more Internet users switch to broadband every year some users still experience slow internet connections through computers, a large portion of the web's population are also still running on the good old dial up connections.
It is therefore unwise to count these out of the equation when you're designing your website, and a very major consideration we have to make for dial up and slow internet connections are users loading time of your website.
Further more search engines such as Google, MSN & Yahoo are now factoring in the loading time so achieve higher page rankings, image optimisation is now a very important factor in the design of your web site pages.
Generally, all the text on your web site will be loaded in a very short time even on a dial up connection and slow computers. The culprit of slow-loading sites is mainly large images on your web site, and it is very important to strike a delicate balance between using just enough images to attract your users and not to bog down the overall loading time of your web site.
You should also go to a greater length and optimize every image on your site to make sure it loads in the least time possible. What I really mean is to use image editing software to remove unnecessary information on your images, and thereby effectively reducing the file size of your image without affecting its appearance.
There are many free image compressors online that you can download and use to reduce your image's file size.
On the other hand, you can opt to save your images in PNG format to get the best quality at the least file size. You can also save your images in GIF format -- the image editing software clips away all the colour information not used in your image, hence giving you the smallest file size possible. However, saving in GIF format will often compromise the appearance of your image, so make your choice wisely!