In some cases, PNG graphics can be a lot smaller than other graphic types, especially if you have solid colors. However, if you do a Save As in Photoshop, you will find yourself creating large, unoptimized PNG graphics.
Watch this tutorial to see how to optimize your PNG graphic, using Save for Web, to make your web site as fast as possible.
Note: Photos are usually best saved as Jpeg, however, it is a similar process when saving for the web.
Optimize Your PNG Graphics was originally found on Access 2 Learn