Thunkable Pro members can now access screenshots for iPhone 8 plus and iPhone X screen sizes to upload to the app store. Unfortunately you cannot get iPad Pro screenshots. I managed to get a hold of a teacher in school who had an iPad Pro and was happy to help. I recommend asking about with people you know if they could help you out. Changing the dimensions of the image will not help as the images look different on iPad and iPhone.
How can i acces to the iphone Screenshots?
Contact a member of staff in the community, and they will direct you further. They will create the screenshots for you