Hello @MARKETING2!
I am terribly sorry for misunderstanding!
Unfortunately, our doesn’t have an option to add 2 different images for desktop and mobile 
Please accept my sincere apologies! So, you are absolutely right, you need to use pictures that look good on both sizes.
This is a wonderful idea and I’m happy to say that we’ve successfully added it to our Wishlist. Please vote for this feature to get a notification upon its release here - Different images for mobile and desktop
Feel free to leave a comment there or any insights you have as it helps us improve our apps.