Many people use images in their slide presentations that are smaller than the size of the slide. Typically the images are placed underneath or away from whatever text is on the slide.
To convey your message in a much more powerful way, you can use a full bleed image. “Full bleed” means making the image bleed off the sides of the slide, taking up the whole space of the slide. Then you just put your text, making sure it has enough contrast, on top of the image as can be seen in the before and after below.
Image Source: stadje