When saving assets for any website, it's important to remember a few things regarding how the image is saved. 

1. If there are no transparent parts of the image that need to display on anything but a solid background, use a web-quality jpg. From Photoshop:

  1. File
  2. Save for Web & Devices (Alt + Ctrl + Shift + S)
  3. Select JPEG
  4. Lower the quality until a noticable difference in the image is visible
  5. Save

2. If the image is large, has transparent portions, and has to display over an image background, export the image as an 8-bit png 

From Fireworks:

  1. Open the .png image

  2. Navigate to the Optimize window (Window > Optimize or F6)
  3. Select PNG 8
  4. Select a transparent Matte
  5. Select Alpha Transparency

These steps alone dropped the file size on this transparent UXi® logo from 47KB to only 18.  When used on larger images, the result is even more dramatic.