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Background Image for Site

Hello Im trying to set an image for my website and im having some difficulties with getting the image to fit the background of the website because it keeps getting cut off on the edges. the second thing is on the background for the mobile it wont stay fixed even though I have it set on fixed when I publish it and visit on my phone the background still moves. I tried to hire someone to help me but they said my sections are off can someone please help thanks.