Six Revisions has a great article about using Photoshop to create web graphics.
The article outlines the following tips:
- Save your graphics with the “Save for Web & Devices” Command
- Consider using web-safe colors
- Consider using the “Progressive Scan” option for JPEG
- Save graphics with transparency as PNGs
- Save animated graphics as GIFs
- Pay attention to “Download Time” information
Read the complete article for more details of each tip.