Change the slide background in Keynote for iCloud
You can change the background color, or add an image to the background of individual slides in a presentation.
Select a slide thumbnail in the slide navigator (on the left).
Click the Format button
in the toolbar, make sure the Background checkbox is selected, then choose one of the following fill types:A preset color or gradient designed to match the current theme: Click the preset color well to the right of the checkbox, then choose a color or gradient.
A custom color or gradient: Click the pop-up menu below the checkbox and choose Color Fill or Gradient Fill. Click the custom color well below the pop-up menu (it has more options than the preset color well) and choose a color.
An image: Click the pop-up menu below the checkbox, then choose Image Fill. Click Choose, then double-click an image file with a .jpg, .png, or .gif filename extension.