Its not a rule. Nor is it a myth. What you are looking to read up on is the reading habits of people in general, typography, and usability.
For a starting point I suggest reading the "Most Popular" section in "Articles" of Nielsen Norman Group (nng.com/articles). They have completed numerous studies on the subject and could explain it far better than I ever could.
To help with the answer I will give my two-cents
, but really you are going to learn quite a lot from that page alone.
Who is going to read this?
What you are essentially having to think about is "Who is my target audience?"
The reason you ask this is because we all have varying levels of reading ability and interest in a subject. You're right, walls of text are extremely off-putting, unless they are in a novel which is where you would expect them. Thats why there is no such "rule" to it, only situations you need to adapt to.
What product are you selling?
If you're selling a car, or a product of some sort you need to understand that people are generally just looking for they key points (tech specs etc...). They don't want to know about the ins and outs of the build process. Thats why a brochure is generally very short on content but includes lots of full colour images. The text won't sell the product on it's own.
You also need to ask where your information is going to be displayed. Is is a web page, because web users scan a page.
How many times have you read through an entire website? Not many, if ever I assume.
So yes, using images is key to breaking up information into readable chunks for the user. However, an image must be relevant for it to be effective so I am not keen on the idea of using anything to break it up.
Understand your content
Another point to think about is having a good understanding of the copy you have written/are using. Identifying key issues and using these as "breakout" sections. Think how a magazine is laid out. They're full of content with a structure to help the reader but they also use breakouts to entice the reader onto the next part. Be that a quote of some sort or a really powerful message within that next section.
As I mentioned, I am very new to the subject myself and cannot explain it as well as the group in the link. In short you should focus on:
- Grids and Layouts
Once you have a basic understanding of these you can then begin to experiment more.
Google also has a useful section on some basic "rules" of typography in Material Design too.