Hey @info70! We don't recommend adding a single header image directly to that slot in the Card template, as it will cause this to happen on mobile when it's set to 'Contain' (i.e. the image will take up the full width of that area, but will not stretch its height — so if the container ends up longer than the image, it will just leave white space below it, as you can see in your screenshot.)
We also have a video demo of the Card template here159 that should help clarify further :) In this case, I'd probably suggest trying a different template and add that header image as its own image block, instead of using the one built into the Card template. Alternatively, isolate the logo from the background and upload the background image into that Card template header (set to 'Cover' rather than 'Contain'), and then add your separate logo file as its own image block on top (this method is also demonstrated in the video link provided, if it helps to see a visual example!).