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Although PowerPoint provides many options for the backgrounds of your poster (slide), you will not be able to print a large format poster with anything other than a white background.
If you intend to print a small copy of the poster (8.5"x11" or 8.5"x14"), you can select a background on the Design tab. You will see many Theme options initially. If you want a picture background, select “Background Styles” to the right of the Themes, then click “Format Background…”. Choose the radio button for “Picture or texture fill” and click the button that says File… Find the picture you want from your hard drive. You will probably want to fade or lighten the image prior to using it as background or you risk the picture competing with your text, but you can also adjust the Transparency using the slide in this Format Background dialog box.
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