Guidelines for Using Images

Consider Image Load Time: Keep total file size on your webpage below 60KB. Pages load faster when there are less images, only use them when absolutely necessary.
Reuse Images: Reuse common graphics such as logos and navigation buttons on multiple pages instead of creating different versions of these common graphics.
Consider the Size/Quality Issue: Although larger image files are better to look at, keep in mind that they slow down the page. Choose the smallest files with the best quality.
Use Appropriate Resolution: Higher resolution means that more bandwith is required. Use resolution settings that are appropriate for web pages. Keep it between 72ppi and 96.
Specify Dimensions: Use photoshop or another graphics application to resize images when needed. Although you can do this by modifying the height and width attributes on image tags, the page will end up slowing down.
Be Aware of Brightness and Contrast: Be aware of how images will look in terms of brightness on various operating systems and even different types of monitors