When you say background image splits in 4, do you mean it tiles (repeats) to fill the viewport?
Background images do not normally resize to fill the screen. If that is the effect you want, see below for example & CSS code.
Yes, it repeats itself to fill the viewport. It is 4 images of the same image.
It doesnt make sense as the background image is as large as the "main page"
Can you show us a link to your page?
Here you are!
There should be only 1 of those images!
That link points to your computer which no one else has access to.
Instead of using Page Properties use Css. Insert the snippet of code below into your pages code directly after the closing </head> tag. Go into code view to locate the closing </head> tag. It will be near to the top of the page and make sure you find the one with the leading slash /.
Replace 'yourBackgroundImageName.jpg' with the name of your image and make sure the image is directly sitting in your site root folder. If it's in a folder named 'images' then add the name of the folder to the url: