How do I replace an image by a video?

Although Video Background, as its name implies, is specialised on backgrounds, if can also be used to replace some content by simply setting the target transparency property as transparent:

Using that technique the video will replace the selected element. It can be used to replace any element but it is often used to replace an image.

Anyway, there's no point in loading a full resolution image if you're going to replace it by a video, so you may also consider setting a blank image instead of the real image.