There are a few workarounds.The most common method is to turn off the grid, remove any movable or temporary tokens such as monsters, PCs, doors, etc.. from the map (you can move them to the GM layer temporary then turn the opacity down to transparency), disable any fog of war used, then zoom to a comfortable detail level and start snapping screen shots. After you make multiple screen shots in your photo program, you can either stitch them back together to make a complete map or just upload the pieces back into roll20 so you can rebuild the map in Roll20. You can also share the map or the pieces with other people this way also but they would be required to upload it to their games.