I'll share with you some ways to do this via my original vbulletin.org article.
This covers the following PageSpeed Rules:
If you have access to Visual Studio 2010 (http://www.microsoft.com/visualstudio/en-us/try) grab the trial edition here[/URL] which should be compatible with the addon), you can complete this step in just a few clicks using this addon
You can also use pnggauntlet which will compress images as well.
This is great to use on new skins/themes and also non animated gifs for Smileys, Signatures, Photo Album & Group Images and Avatars that your forum stores on your server!
Please note that both of these methods produce lossless images, meaning that even though it uses various methods to decrease file size there is absolutely no visual loss in quality of the image.
For non-windows users, I suggest Imageoptim &