Email marketing is a medium of clarity and responsiveness, not graphic appeal. Your words and simple images do the selling, not elaborate borders, shading, and colored backgrounds.
There are several factors which can prevent email messages from being delivered; and sending a large message file with detailed background images is exactly the wrong thing to do. In addition to causing most of the messages to be delivered to junk folders, a detailed background image will take seemingly forever to load on mobile devices and lead to greatly reduced open rates. (Typically 30% to 50% of those who view a message do so on a mobile device.)