When you have the Power-point slide open on your screen to your liking, hit the 'Print Screen' key on your keyboard. Then open your 'Paint' program and hit 'Paste'. The resulting image can then be cropped, resized, etc, and then saved as a jpeg or gif for uploading to your photo-server.

You should be able to save an individual slide as a jpeg though, by right-clicking on the selected slide and 'save-as' before starting the show.