Yes, you can upload any image you would like as a background image to your video (included in all pricing plans). You can crop them in the app after uploading and the recommended resolution is 1920 x 1080.
For inspiration: we've prepared 30 FREE video slide templates that you can use. Simply download them, tweak them and then upload to Synthesia when creating a new video.
More social media backgrounds: here's some more FREE Google Slide background templates that might fit your social media videos.