This doesn't happen very often, but if it does... here's a potential fix for it!
If your image is from our library, please try to delete the scene and add it in again to the background.
If you uploaded the image, delete the upload and upload again.
If this doesn't help, please reach out to support. We'll look it over and explore all possible fixes so your video can be set. However, if it's pretty urgent, we recommend choosing different background options, as a fix may take a while.