@angelz Have you read my post on Medium? You should be able to get a separate image upload for mobile images. I didn't test this with multiple slides, however, I would recommend to only use one slide after all..

This question (and answers) helped me a lot. I wrote this into a small blog post for easy reading: https://medium.com/@nickdaelemans/using-a-mobile-header-image-on-shopify-be7e4cae60f9?sk=03916a16452234570355b2210dee120b