OK I see what you mean. Basically the image isn’t keeping it’s exact ratio across all screen sizes - although it is quite close.
I would have thought that putting the image in a supplied Foundry Image stack at full size with no margins / padding etc would work. If not, then you need to get some advice from Elexir - Adam’s very helpful in my experience.
There is an older thread on this general subject here: The sizing of (Responsive) headers/banners