I doubt that the dashboard area, itself, is something you can edit easily for your instance. You can put an image on course tiles. It's a setting for each course. I created a blueprint course that pushes an image in the tile, but that can be changed by the faculty member. The tile color chosen by the student overlays the image, so the colors of the image may be muted or appear altered.
Here's the FAQ: How do I add an image to a course card in the Dashboard?