This how-to was done with Office 2007. Menu locations and screenshots may look different in your version, but the idea should be the same.
Open up your PowerPoint file as you would normally and navigate to the Design tab. You should see a button Page Setup. On Mac this can be found in the top Mac bar under ‘File’.
In the dialog box that appears, enter the size Custom for the slide size. Enter 20 inches width and 11.25 inches height. Make sure that the orientation is set to Landscape.
In the top left corner click the Windows icon and go down to Save As.
Enter a file name. It will create a folder with this name and put all the slides in that folder numbered. In the Save As Type dropdown select PNG Portable Network Graphics. You can quickly jump to it by clicking on the dropdown and hitting P on your keyboard. Click save when you are finished.
A popup will appear asking you if you want to save the current slide or all the slides. Select Every Slide.
You now have a folder of PNG images ready to move into your video editing program of choice.