The key-visual is not directly a part of logo design, but under certain conditions it is a very effective add-on to the logo. The key-visual is an image motif that is used in campaigns in order to enhance brand recognition.
Often, the key-visual is more strongly associated with the brand than the logo itself, so the development of a key-visual may be very important. The key-visual should be easily and quickly grasped.
At the stage of logo design, you should already contemplate which key-visual could be used in conjunction.
Again, you should pay attention to the following aspects:
The key-visual should be simple and understandable.
It should be unique and unmistakable.
It should be usable in all media.
It should address the feelings of the target group.
Analyze the competitors' key-visuals and don't design a plagiarism of them - create something new and better.
Examples for good key-visuals: Michelin Man and the Schwäbisch Hall Fox.