Based off of those pics, if there is no other electrical panel, you'll need to upgrade the entire breaker box. On a more common home, there would be a main panel with the ability to add more breakers to it. Your current panel has no available slots and piggy-backing off of the 50A circuit could lead to issues later (not to mention it would trip if you're trying to charge while the breaker is also actively powering something else) . Unfortunately, you'll need to spend a few grand if you want to do this properly.