For a pieced together map element, your only real option is a screen capture. This may require you to piece it together outside of Roll20 if it is very large. As a hint on how to manage groups (which I only recently learned myself), the trick is to select the group with a click, and then click and drag. If you drag with that first selection click, it will only drag one element of the group. A bit counterintuitive, but it does give you the option to move things independently on an ad hoc basis, like to re-aim a ballista token).