Rémi Parmentier wrote about an experimental technique that allows background images to work in the Gmail Android App for Non Google Accounts (GANGA).
You see when Gmail announced support for embedded styles and media queries in the Gmail App, they neglected to mention that the support was only for Google accounts like Gmail or GSuite (formerly Gmail for Business). Emails retrieved using the Gmail App with non-Google accounts on Android, still lacked embedded styles support. What was more alarming is that GANGA also lacked background image support! Yup BOTH CSS and table background image.
Rémi’s technique doesn’t use CSS background-images or the background attribute in tables but involves a positioning technique to slide images under content using a technique pioneered by Mark Robbins.
Read more about how its done on Rémi’s blog.