There is a concept called 'Infographics' that uses lots of colourful diagrams and graphs and artwork to visually depict some set of concepts and tries to explain the ideas through pictures. I recommend these Infographics to any person trying to learn some complicated principle in Math, say. I was wondering if there are any easy-to-comprehend infographics for hard subjects like Category theory or Topology?
For topology, Peter Saveliev has a draft online book
True to the title, it is heavily illustrated. While there is certainly plenty of explanatory text, the large number of pictures and diagrams could be seen as a form of infographics.
For complex variables, there is the famous "visual" book
Tristan Needham, Visual Complex Analysis
The slides of Dror Bar-Natan are excellent examples of such mathematical "infographics".
In addition to Needham's famous book mentioned in Gerald Edgar's answer, there is
Visual Complex Functions: An Introduction with Phase Portraits by Elias Wegert.
The book is richly illustrated with colour diagrams of complex functions, especially the "phase portraits" mentioned in the title, as can be seen on the author's website for the book.
Traditionally (say up to the last 40, 50 years?) images were extremely expensive to produce and distribute (line graphs had to be drafted by hand, other images had to be created by expensive methods, colour meant special pages in the book), and thus used sparingly. Creating, distributing and viewing videos was much worse. As a result, people streamlined methods to make their point without images. That is hard to unlearn.