I hope I did not sound rude earlier but you know the Redskin vs Cowboy rivalry. When it comes to plumbing the potential problem is leaks or water damage. When it comes to electricity it is fire or death. I am not being pompous I just take my job very seriously. Your panel is G.E.(thanks for the picture) For the age of the house that work is not the most visually impressive. I don't like the way they lumped all the grounds together, and only used two connectors for all the circuits. What if you ever had to remove one wire? It is a stab in type panel, and there appears to be room for another neutral and ground. Thats why I mentioned that earlier, you do not want to loosen a screw that has an existing neutral under it. Both the neutral and ground go under the same section, but the neutral and ground must be under seperate screws by code.If you throw the main you will shut down everything below the main breaker. However the two large wires going to the main breaker will still be energized. I don't know about Texas but most everywhere else you are not allowed to pull the meter. The good news is you do have room for an additional circuit. The bad news is it looks like the panel is recessed in the wall, so getting the wire in the panel with a connector might involve damage to the sheetrock. I would not recommend this work to a begginer with electric, thats just me being responsible.