Page Background is your Experience page's direct background. The background look will vary depending on Template.
(The Planet Image on this screenshot background)

To set the Page background

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On the Experience Edit page hover the mouse over the Background Photo/Video tab on the Page background card to see three image uploading options:
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Drop image here: the feature enables a user to drag an image from a PC folder.
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Upload from computer: enables a user to add an image from computer.
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Select from gallery: enables a user to add an image from the app Gallery.
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Choose an option and upload an image.
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Click Submit button in the page right upper corner to save the changes.