I beg you, if you are a developer of an open source app or program - add screenshots of your app to the README file. When looking for the perfect app, I had to install dozens of them just to see what the user interface looked like and whether it suits me. This will allow users to decide if the app they choose will suit them… Please, don’t think about it, just do it…
A README file is usually comprised of text.
Other than that - usually if it has a webpage, it has some screenshots.
README markdown files allow for inline image links to be “expanded”