hi @Nicolas_Stockmans here is one link from forum
But feel free to find more articles when use “images turn” keywords
https://discourse.webflow.com/search?q=images%20turn
Simply you should use some editor to turn these to required orientation. The good practice is do that when you optimise your images for web. You can use any editor that is included on your OS to do this job or use any editing software or online services.
Hope that helps