If you add an image to your slide, you can use the Auto Color feature to make the background color match most common color in that image. You can find this feature in the color menu (the drop icon) in the editing mode of your slide.

When you have multiple images in the slide, Auto Color will pick the first image in the slide as the color center. To change which image a slide uses to autocolor, you can drag the image you want to use to the first position on that slide and Auto Color can will do the rest.