header image doesn’t crop

Hi,
I have some issue uploading the header images.

I’m using Savona Edge.
After uploading the image, the function of ‘cropping’ doesn’t let me crop anything and the images get uploaded cropped anyway.. Even if I skip cropping the image looks cropped anyway.

On the customizing page of the header it is written:
‘Click “Add new image” to upload an image file from your computer. Your theme works best with an image with a header height of 0pixels — you’ll be able to crop your image once you upload it for a perfect fit’

I think it is probably a problem of pixels but I have no idea how many pixels should my image have to be able to crop it!

Thank you if you can help

The page I need help with: [log in to see the link]