With Promomatic you can upload some app screenshots, add a caption and you're good to go. Right now App Store, Play Store and Product Hunt-friendly image sizes are supported. But I plan to add more image sizes for different use cases.
I'm also really interested in the automatic / assisted design space. Helping people do design faster or helping people with no design skills to create great-looking designs. It's quite likely I'll continue exploring this space in my 12 startups challenge. Long term, it would be super interesting to apply to ML to this space.