Certain scenes allow you to import a JPG or PNG still image either as a background (like the “Presentation” scene) or as an element within the scene itself.
This features is indicated by the presence of the Import image icon:

Click the Import image icon and browse your computer’s hard drive for a JPG or PNG image.
If you want to place your characters in front of video, then add a new Scene and choose a green screen scene. You then can edit your finished MP4 video in another video editing application.