The background of a the site is generally a single image that, if you want to keep the size down for load times, is repeated vertically or horizontally. For what you want that would typically mean a picture as in the following links.
Here is the site.
Here is the background image.
If look at the dimensions of this image they are 1200px by 7px and the image repeats vertically.
You would have a narrower image in your case and you would have to position it with CSS as explained here
That is one methodology. There are ways to use two images or a single image can be used but you have to be careful of how large the file size is otherwise you can run into long page load times as mentioned previously.