Image Guidelines

Consider Image Load Time: Images take time to load. Suggested maximum file size for both the web page and all the media files used by it is 60KB.
Reuse Images: Once an image from your site is requested from a web page, it is stored in the cache on your visitor's hard drive. Subsequent requests use the image off the hard drive creating faster load times for images that are reused.
Consider the Size/Quality Issue: Try and choose the smallest file that gives you appropriate quality.
Use Appropriate Resolution: Web browsers display images at relatively low resolution - 72ppi or 96 ppi. Be careful when taking photos or scanning and use a resolution setting appropriate for web pages.
Specify Dimensions: Always use the accurate height and width attributes on image tags. This will allow the browser to allocate the appropriate space on the web page for the image and load the page faster.
Be Aware of Brightness and Contrast: Gamma refers to the brightness and contrast of the monitor display. You should be aware that images will look different on various platforms because of gamma.