Summary on Graphics for web
The key to optimizing web graphics is to minimize their size, number, and quality, and maximize cropping.
Combine adjacent images and use client-side imagemaps instead. Substitute CSS-based rollovers for bandwidth-hungry image-based rollovers.
Replace graphic text with styled text. Finally, to minimize HTTP requests, reuse images with the same URL.
With all graphics, you can't go wrong if you do the following:
Eliminate and combine your images to minimize HTTP requests.
Convert graphic text into styled text.
Maximize cropping and minimize dimensions.
Use the right image format (JPEGs for photographs).
Use interlacing with care (avoid interlacing on smaller images and most PNGs).
When capturing photographs, use a stable platform, (tripod, image stabilization)
Minimize noise, smear, and complexity.
Use weighted optimization to reduce less important areas and blur backgrounds (for JPEGs).
Minimize colors (bit-depth) for palette-based formats.
Use a high-quality quantizer on true-color originals and limit dithering.
Set your image resolution to 72dpi.
Specify the actual height and width of all images.
Use grayscale where possible.
Use thumbnails linked to larger images.
Experiment with different settings and programs.