background image width

Site URL: https://ayarresearch.org/

I am new to version 7.1, and would like to have the background of my page have white margins.  I am able to do this by increasing the site margins in the spacing section. However, here's the issue - when I put in an image, I have the option of making it full bleed or create an inset.  The full bleed exceeds the site margin, and goes until the width of the browser window, where as the inset option creates a bottom border (3 insets).  I would just like the sides to be inset, ie - is I would like the background image width to span only until the site margin. Please advise and thanks in advance.