In order to change the large image on the home page of the Rachel theme, sign in and edit the home page.
Scroll down to the Themify Custom Panel > Themify Builder tab and click on the Options (pencil) button at the top right corner of the row. This image is set as the background of that row.
Then go to the Styling tab > Background drop down and you will see the option to change image.
You can also change this background area to a video, slider or a solid color.