Why does my craft theme banner blur with two images?

Store Design
1

Hello,

I have a banner with two images:
one image for pc, and another for mobile. the problem is that the pc image becomes blurred because of the second image and i have no idea why. when i delete the second image, the first one becomes clear again. Does anyone have a solution? url: lapin-calin.fr

2

same problem here, i have used the solution from this post: https://community.shopify.com/topic/1362462 in order to get a separate banner for desktop and mobile. But the image quality is really bad now.