Changing the top Header image

Hello! I made some changes to a website : www.spff.ch. The size of the top Header image (titel of the website) is now 1000 x 200 px. When browsing full screen on my computer, everything's fine. But when browsing on my smartphone, the image is deformed. How can I fix this to get the image resized proportionally (horizontal and vertical), without any deformation? Thanks in advance for your help! Marc