If you are using CS2 go to File > Save for Web and do the following:
- Step 1. Click on image size
- Step 2. Change the longest side to 800 pixels and click Apply
- Step 3. Move the quality slider until the file size in the bottom left corner is below 160kb
- Step 4. Save
I've tried to illustrate it here, please excuse the child like squiggles!
