An effective visual considers all aspects, ranging from picture, colour and even font. This is a good example from Korea.
A good visual is one which communicates something very quickly. The above poster is a very good example from Korea, where there is very little text to maximise the impact of the visual.
The purpose of poster is to remind people the difference between two very similar phone numbers. In Korea, the 110 is the Government information call centre, while 112 calls the Police in an emergency.
The facial expressions depicting the two numbers clearly describe the nature of each number, while red shows which is the emergency number.
Keep your visuals simple and clear for it to have the most impact.