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How to set Use Black Grid Backgrounds (800x600 Resolutions) picture as a slide background?
1. Click the slide that you want to add a background picture to.
To select multiple slides, click a slide, and then press and hold CTRL while you click the other slides.
On the Design tab, in the Background group, click Background Styles, and then click Format Background.
2. Click Fill, and then click Picture or texture fill.
3. To insert a picture from a file, click File, and then locate and double-click the picture that you want to insert.
To paste a picture that you copied, click Clipboard.