You may want to consider building this as a Motion Template. To learn how click on this link: Building Motion Templates.
As for your question. You can swap pngs by, selecting the new image in the File Browser. Then drag it on to the old image in the Timeline or Project Pane. Wait for a hooked shaped arrow and then let go. It will swap the images.
Or you can select your image. Go to the Properties tab. Go down to Media and you will see a box that says “To”. Select it and you can choose any other image in the timeline.
Hope this helps and best of luck.
Stephen Smith
Utah Video Productions
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