I am guessing from the photo this is a cube type tank? Where you place the heater in the tank isn't really important as long as the area has good flow around the heater.
When I had freshwater tanks, I tended to place the heater in a line with the outflow of the powerhead. The powerheads in my tank, normally 2 in my 75 gallon, were placed to cause a circular motion (moving left to right at the front and therefore right to left in the back) of the flow in the tank and water column.