Is it viable to put an engine at the rear of the plane like in this photo?
As you look closely to the picture, there are also engines located under the main wings. Thus this is a trijet, a concept that has been used in the past by for example the MD-11, Lockheed tristart and Boeing 727. The main reason for choosing a trijet configuration was the ETOPS requiring twin engine aircraft to always be within a certain range of an airport. This was in a time when engines where less reliable. With three engines the aircraft did not have to bother with ETOPS regulations allowing it to fly routes impossible to fly for twin engine aircraft. But as engines became more reliable and the ETOPS got lengthened the trijets became redundant.

A big downside of the trijet is the structural complexity of an engine in the back, the fuselage needs to be reinforced to withstand the extra forces. From a maintenance perspective it is difficult to access the middle engine because it is high of the ground requiring a stair for visual inspection. Then there is the economical aspect, if you can do it with less engines it will always be more fuel economical. The next iteration of 777 can carry as many passengers as the 747 with half the engines.