Show a warning pop-up if a user clicks the Submit button without drag and dropping all the images to a zone

Hello Nicolas, 

Thanks for reaching out!

It would be much easier for the community to help you if we could take a look at what you've already built as reference. Would you be willing to share a copy of your project file here or in private by opening a support case? If you decide to share it privately, we'll be sure to delete it when we're done!