While Apple's Keynote software gives you several backgrounds to choose from when you select a theme for your presentation, there's no reason to limit yourself to those options. After selecting a slide, click the "Format" button to access the Background options or create your own background by pasting any image into the slide. You can change slide backgrounds one at a time or apply the change to all the slides in your presentation by editing the master slide.
Using Your Own Images
Open a Keynote presentation you have already started or create a new one using any theme. Select a slide in the Slides panel.
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Locate any image you want to use in your presentation as a background image. Right-click an image in Finder or an image on the Internet and select copy. You can also open a picture in Preview and select a portion of the image by highlighting it and then pressing "Command-C" to copy the selected portion of that image.
Right-click the background of a slide in Keynote and select "Paste" to paste the image into the slide. Right-click the image in the slide and select "Send to Back" to make it the background.
Drag the corners of the image to resize it as needed for your slide. To move the image, drag the center of the image until it's located where you want it.
Add additional slides to your presentation using the same background image by right-clicking the slide thumbnail in the Slides panel and selecting "Duplicate."
Background Colors and Textures
Select any slide you want to change in Keynote by clicking its thumbnail in the Slides section. Locate the "Background" section in the Slide Layout panel. There are three menus to choose from: a Background menu, Color Fill menu and Color menu.
Click the box located to the right of the word "Background" to select a Color Fill, Gradient Fill or Image Fill. Color Fills are simply solid color backgrounds. Gradient Fills can be adjusted using the controls to make the color become lighter or darker from the top to bottom or side to side. Image fills give you textured colors that resemble coarse paper; they can be tiled or stretched to vary the texture as needed. Click the "Choose" button to select your own image, which can also be tiled or stretched as needed.
Click the "Color" menu to customize your own color by clicking the "Color Wheel" or to select from any of the several colored crayons that appear on the screen. Click the "Color Fill" menu to customize the color as well; it includes Gradient Fill, Advanced Gradient Fill, Color Fill, Image Fill, Advanced Image Fill and No Fill.
Changing the Master Background
Select any slide thumbnail in Keynote's Slide section that uses the default master slide you selected when creating the presentation.
Click the "Edit Master Slide" button in the Slide Layout Section.
Right-click the background to paste an image in the slide and move it to the background as you would a single slide, or select any of the Background options in the Master Slide Layout section. These options work the same as when editing a single slide.
Click the "Done" button beneath the master slide. The change is applied to every slide in the presentation using that master.