Well if you mean backgrounds as in pictures id suggest getting your favorite pictures that fit the subject and going into PhotoShop or some program like that (besides basic paint) and lightening or darkening the images so they arent distracting.
if you want a background made for you please send some pics and instructions to [email hidden]
Actually, you don't even need to use Photoshop or anything like that. You can do it straight in Power Point. Just find a picture you like, resize it and crop it to fit the whole slide. Then right click and go to Format Picture. That first page should have a place for Brightness and Contrast. If its a color picture you can make it grayscale so it will be black and white or you can leave it color. All you really need to do is raise brightness and lower contrast to how dark/light you want it. I recommend doing this in the Slide Master so it will just automatically go to all slides. Good luck