Not sure if this addresses your question directly, but I wanted to remind you that Email CSS must be inline. It cannot refer to external sources.
So, for example, you would include the <style> configurations at the top of your email definition. (I have done it several times)
This makes the background color of my email gray. In order for your email to use the desired background image, you would also need to ensure it was publicly available the recipients, and use a full URL, not a relative URL.
/* NOT THIS */
/* NOR THIS */
/* THIS! */