Keep the images simple. Physical image size should cater to small monitors. 400 x 600 pixel should be the largest image size. Avoid excessive detail. Images should support the content and not distract from the purpose of the page.
Web sites have three general types of images.
Each type of image has its own design requirements. Images may have any design. The important concept is to have all of the elements in the web site working together. Here are some general guidelines. Be aware of image size.