"A picture is worth a thousand words" as the saying goes. People have been visually recording history since pre-historic times. Whether cave drawings, sketches, artworks, photographs, or videos, an image can instantly portray an event or person in time. They literally let us see what has happened and allow us to view history in a different way than textual documents.
As with any type of source, we also have to be skeptical when using images as they reflect the perspective of the person creating the image. We know that artists can take some license with their work and also historic paintings have been created after the event, so the artist may not have been an eyewitness. Also, photos and videos can be edited to present what the creator wants us to see.