You need to edit the page with the Supporters page template and scroll down at the Individual Page Settings. The Cover Image field allows you to upload a background image.
You can also use a video. The External Video Link allows you to add an YouTube video. The Video Background MP4 / WEBM / OGG File allow you to upload your own internal video. Please note, you will have to upload the video in all 3 formats. Please note, if you have an external video added, it will override the internal video.