You can upload any image/video you would like as a background to your video (included in all pricing plans). The recommended resolution is 1920 x 1080. The video size can be up to 50MB and the duration up to 5 minutes per slide/clip.
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.
Custom background image
Custom background video