Using Different Backgrounds within one Presentation
Although you only have two background designs automatically supplied with the Masters (counting both the Slide Master and the Title Master), you can have any design you want on any slide. From the Format menu, select Background. Check the box that says "omit background items" and this will make the slide ignore the Slide Master's design. You are now free to add whatever design you want to this slide. If you want to do this to many slides at once, go to the Slide Sorter, select the slides, and then use the Format menu command. Remember though that if you choose to do something like put a photographic background on many of your slides instead of doing it once on the Master, that your file size may increase dramatically.