You can replace some placeholder images with photos, movies or audio files. When you drag a movie or audio file from the Media Browser to one of these universal placeholders, you will see a green "plus" icon before you release the mouse.
However, there are placeholder images in some templates that you can't replace with video or audio (if you try, you'll see the file link arrow shown below).
You can replace these placeholders with photos. You can also delete the placeholder image and then drag a movie or audio file to the canvas.