It is true that usually there are no figure in introduction.However, I think that if this figure is essential for a better understanding,you can include it!
In my opinion, there are one crucial rule in a publication... all the readers must understand!
It depends, where it should be published... if can restrict the extent, decrease the attention of users etc. In general, it is better to use a specific and self-generated images rather than assumed illustration. Anyway... the primary aim is to make the reader understand the presented topic and this is the reason why figures are usually very suitable. Of course it is necessary also to ensure compliance with licensing rules of the image.