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…
That’s one option, or use imgur.
Update them if your UI has significantly changed or does not adequately represent the final product.
If having a light/dark theme is an important feature or highly requested feature for your project, it would be nice to show it off.
Screenshots can, most of the time, get away with showing just the default configuration. Share what a user would see when opening your project for the first time, and assume they used the default configuration. Optionally, if you offer a lot of customization, show what it could look like if someone spent a good amount of time personalizing things!
Please don’t use a external image host, have it live with your code in /docs