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…
While I get the sentiment, historically, readmes have been text only, and should predominately focus on usage options, not a sales pitch. Today in GitHub, these files support markdown, but the level of effort is probably two orders of magnitude higher than a text readme alone.
Think of a readme file on GitHub/distributed with the binary more as a man page than a proper website.
So why not add a ‘Screenshots’ section with hyperlinks to PNG files?
Or, hell, just add a “screenshots” folder to the repo and mention its existence in the readme.
The best solution is to create an issue and attach the pictures there. You can then link in README and not bloat the repository.