Images are the obvious culprit to slow page loads. But, by strategically placing optimized images, you can achieve the benefits of visual stimulation without the cost of slow download times. You can realize significant file reduction gains by reducing the actual physical image size. In fact, overtime you reduce your picture size, your image size will be reduced by a factor of 4. Therefore, you should give special attention to using images that are as small as possible.
Image quality is the second image optimization factor to focus on.
Quality is always subjective. Programs like Macromedia Firework's will let you compare up to 4 different image views and let you pick the one that meets your need. Some images will load faster. Follow this link for more guidelines. Here's how you do it .