Hi there,
We are working on an official announcement of this feature (once issues to do with updating apps to be compatible with iOS 13 have largely subsided!), but if you message me with a link to your app that begins with x.thunkable.com/projects/, we can generate iPhone screenshots for your app.
Please include:
The email address associated with your Thunkable PRO account
An email address and password that we can use to sign into the app (if applicable)
Thanks,
Jane
Edit: you can access the screenshot service through this post