Since you have a fixed width (1800) you could put in a fixed padding of 1800 - 900 / 2 or 450. But if the screen is less than the 1800 then the image will not appear centered.
If you wanted to do it more that css way then add margin-left: auto; and margin-right: auto; to the image container. This also has the problem that it won't appear centered if the screen is less than 1800 wide.
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