Many of the backgrounds come from the ones that the creators of Scratch made. You can get them by clicking on the stage in the sprite list (the dark gray box below the stage), then clicking on the backgrounds tab. Finally, click the import button. You will get several folders where you can pick different pictures from.
Also, you can get backgrounds from online. Just save them to your computer, and import them using the same method above.
Paulmedwal
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If you have photos you want to use as backgrounds, it is worthwhile to scale them to 360 by 480 in a program like Photoshop Elements, rather than relying in scratch's scaling of the image.
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