Considering that your thermostat is not included in this picture, what is that cord? Does the rock hide heat up? If so find a new hide, or just completely cut the cord off. Rocks that heat up can reach very dangerous temps. If it is the only source of heat the snake will curl around it in an attempt to warm itself. This will burn your snake.
You need a UTH, under the tank heater. A heating pad will work just fine.