If you don't find a predefined theme that you like, you can either upload your own graphics or change slide background colors to create a theme. Before you upload your favorite graphic, think about the overall visual effect that you want your theme to have. Overpowering or strong graphics pull attention from your information and can distract your reader. Light solid colors with relevant pictures work best.
Keep in mind as you're considering which images to use Google Docs requires uploaded background images to be 2MB or smaller and no more than 6.25 mega pixels. After you choose the images that you want to use, the next step is to upload that image and set it as your background.
- On the left side of the screen, you will see a thumbnail of each frame in your presentation.
- Click the frame
- From the menu, select Edit - > Change Background
- The Change Background window will open.
- In this window, select Insert image

click to enlarge
- Click Browse, and select the file you want from your computer.
- To apply the image to every slide in your presentation, select Apply background to all slides.
- When you're finished, click Save.