To add a picture or video to your asset library and set it as the background in your video, please follow these steps:

1. Upload the desired picture or video into your asset library. You can do this by selecting the "Upload" option and choosing the file from your device.

2. Once the media is uploaded, navigate to the canvas in your video where you want to set it as the background.

3. Select the uploaded picture or video from your asset library.

4. Right-click on the selected media and choose the "Set as Background" option from the context menu.

By following these steps, you will be able to seamlessly add your desired picture or video to your asset library and set it as the background in your video.