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:
- Save for Web & Devices (Alt + Ctrl + Shift + S)
- Select JPEG
- Lower the quality until a noticable difference in the image is visible
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
- Open the .png image
- Navigate to the Optimize window (Window > Optimize or F6)
- Select PNG 8
- Select a transparent Matte
- 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.