I am asked frequently by clients how to best optimise images for their website. The best way is to create a photo shop file with the right dimensions. Then drag and drop the image into the template, resize and position and then save for web use trying to get the kb size down as much as possible without loosing image quality.
Gif or PNG work best for images that have large flat areas of colour such as graphics. Photos or images with gradients are best saved as jpeg’s. This video tutorial shows how it is done.