1. Select the slide you want to add an animated background to, then go to Insert > Pictures, and choose either Photo Browser or Photo from File.

2. Select the picture you want, and then select Insert. You may need to resize the picture to fill up the slide.

3. On the Picture Format tab, on the far right end, select Animate as Background.

   A dialog will open with a preview of your background picture.

4. Click on the parts of the picture you want to have the movement of your animated background focused on. When you're done selecting points of interest, click Apply.

5. To see your animations in action, view the slide in Slide Show mode.

   To edit your animations, select the background picture and select Animate as Background again, and repeat step 4.

You can add text and other content to the slides over your background images, and they'll remain in place while the background moves behind them.

Animate as Background is only available if you have PowerPoint for Microsoft 365 for Mac, PowerPoint 2021 for Mac, or PowerPoint 2019 for Mac.