If you must use Roll20 to build your map but want to have a backup, do the following. Create you map without any of the movable items such as doors, monsters, and other things. Just the environmental or stationary stuff such as the walls, floors, river or trees (if outside). Disable the grid. Take a screen cap of your map. If the map is small enough to not need to be zoomed out then go for it. If the map is large enough that you must scroll then you will need to take screen caps of a section, scroll, then take another, and repeat till done. Open your preferred photo program and create a jpg when you take a screen cap then you can either stitch them together and create your map whole or keep them in sections so you can upload them if the map is larger than the upload limits. That's the majority of the steps to copy your maps that you built in Roll20 on to your computer. Other people might stop by and expand on it more.