Six Revisions has a great article about using Photoshop to create web graphics.

Graphics in Photoshop: 6 Beginner Tips for Web Designers

The article outlines the following tips:

  1. Save your graphics with the “Save for Web & Devices” Command
  2. Consider using web-safe colors
  3. Consider using the “Progressive Scan” option for JPEG
  4. Save graphics with transparency as PNGs
  5. Save animated graphics as GIFs
  6. Pay attention to “Download Time” information

Read the complete article for more details of each tip.