Edited June 8th:
Now that "How to Change Triangular Numbers into Square Numbers" has been published, here is the link.
Here is a triangle made of four rows of pennies.
This image shows a pair of figures mapped to a third shape.
If you had not read the captions, would you see the relationship between the second and third images?
Create your own images when necessary. The article will have another image found on the web. I simply could not imagine how to search for anything like the last two.
Even as simple a tool as Microsoft Paint crops and annotates images taken with a digital camera. If I had remembered the "crop" feature, I could have taken less fuzzy images in the first place.