If that background is the same size as your room (2000x8000) then it's way too big to fit on the texture page size that is selected by default, which means it will get halved by GMS repeatedly until it does fit.
Change your texture page size in Resource Tree > Options > Windows > Graphics.
However that image is much too big and you should resize it - the most common full screen resolution is 1920x1080, so for a single screen background you can see that 2000x8000 is much too big and a waste of resources.