In the readme.txt file, the “Screenshots” section is a special section. The WordPress plugin repository checks your Assets folder (or the folder containing readme.txt) for images named in a certain way and corresponds them to the entries in this section. I don’t believe it is possible to attach images in the same way in any other section.
Markdown does provide a syntax for including images:
However, I tried to make this work in the WordPress readme.txt validator, and I couldn’t. It looks like the plugin repo strips these out, but I haven’t actually tried it in a plugin’s readme.txt and committed it to see what would happen.