When a screen projector projects the color black, it projects nothing. Think about this when designing slides. If you have an image with an aspect ratio that is different from a regular slide (4:3, 16:9) and it is not possible to crop it without damaging its visual impact, make the bits of the slide that are not covered black instead of the default slide background color you are using. Once on screen, the black border will blend in with the area outside of the projection screen.