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…
One thing though: I’m likely not to stop and consider looking closer at an app if I can’t judge if it’s going to be what I’m looking for. I’m not going to go over random GitHub repositories and create screenshots for their projects. So if the assumption is that the user contributes screenshots I don’t think it will ever change anything for the majority of projects.