So this is a strange one.

Since Gmail has started loading images by defult they are now supporting background images. However if you disable images as defult in the settings the backgrounds are stripped (even when you click to show images).

This applies to emails that are already in your inbox. So I inspected element with images off by default but turned on in the email and it says background-image: url(); then I turned images on by defualt and inspected element and it says background-image: url(my-image.jpg);