When designing graphics for the web, you must consider different issues than when designing graphics for print.
To help you make informed decisions about web graphics, keep in mind the following three guidelines:
Use web‑safe colors
Color is often a key aspect of artwork. However, the colors you see on your artboard aren’t necessarily the colors that will appear in a web browser on someone else’s system. You can prevent dithering (the method of simulating unavailable colors) and other color problems by taking two precautionary steps when creating web graphics. First, always work in RGB color mode. Second, use a web-safe color.
Balance image quality with file size
Creating small graphics file sizes is essential to distributing images on the web. With smaller file sizes, web servers can store and transmit images more efficiently, and viewers can download images more quickly. You can view the size and the estimated download time for a web graphic in the Save For Web & Devices dialog box.
Choose the best file format for your graphic
Different types of graphics need to be saved in different file formats to display their best and create a file size suitable for the web.