Within the Campaign Manager, select the Map Tool to update the background image for the currently selected stage.
The following dialog will show:
Once a background image has been selected from your computer, you will be presented with options to resize the image. Resizing the image is recommended to reduce 1) the size of storage required for your background and 2) improving the performance for rendering the gameboard.
For a typical map, with a 5' square, 1500px on the smallest size will generate the best quality map.
Once you have your choices selected, use the Upload button.
Other options from this dialog include setting the battlemap Fog (and resetting it if needed) and removing the existing background map.