We having been optimizing all aspects of Active Forums for the upcoming 4.2 release. One area where you will see a big difference is with the images associated with themes.  Active Forums 4.2 will use CSS Sprites for nearly all aspects of UI images.  I think you will be very impressed with the improvement this will have on your site.  This also means that you can control all images from the CSS file.  All of the included images will be png files.  However, if you prefer to use gif files you now have that option. 

Right now the themes are built with only three sprite images.  The image below is an example of the images for the default theme.  If time permits, we would like to include a couple other color schemes.  This will also make packaging additional themes much easier.