PowerPoint doesn't have a gallery of ready-made watermarks like Word has, but you can still manually add an image, like a company logo, to one or all the slides.
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To add a logo to all the slides, Select View > Slide Master. Scroll to the top of the thumbnail pane on the left, and select the first item, the slide master.
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Select Insert > Shapes, pick a shape and then click and drag to draw the text box on the slide master. It should be about the same size as your logo. Note: If your logo is circular, use a circle shape.
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Right-click on the shape > Format Shape to open the menu.
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Under the bucket icon, select Fill > Picture fill > File and insert your logo image. Adjust darkness of the logo with the Transparency slider.
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To remove the border around the image, select the logo. Then right-click > Outline > No Outline.
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Exit Slide Master. All slides except title pages will have the logo.
Source: Microsoft