Iris here from Shopify Support. Just in case you were not aware and do speak Dutch, this is our Dutch forum and any question can be writen in Dutch. For questions in English it is properly easier to consult our English Forum, as this is read and answered by a much larger group than here in the Dutch forum. By the way this is a great question but it does fall outside of my expertise, as we gurus are not trained in coding and when the theme is also a third party theme I get completely out of my comfort zone.
I think I might still be able to push you into the right direction for an answer, as I have found the same question on our English forum, which got answered very well. See this Forum question: Different images for mobile than desktop web.
Hope this was usefull and good luck.