It wont be. Split theme is designed to have a background image that does adjust to some extent but does not fully scale down to a smaller image of exactly the same proportions. There are several themes which take this approach - I can’t recall why but there are technical reasons.
You either need to be careful which theme you use or alternatively use a blank theme such as Foundry / foundation where you create your own header using an image stack.
There is another important issue here however. You have put ALL of your key information text into an image and this is bad for both search engines and user experience as users can’t ‘click to call’ or copy the text.
If you keep with the Split theme, try adding maybe an abstract image and use the text area supplied for a main title / sub title as appropriate. You should then add other info like address and phone numbers as text below, ideally in a 2 or 3 column stack so it behaves well on mobiles.
Do shout if you need further help.