Update as of 2019. While there are issues with adding background image from inline image on the email (at least I did not find a way to make it work). Actually css-background image works fine on at least some elements as long as they are absolute urls to resource, my snippet that works on gmail as of June 2019:
<table width="100%" border="0" cellspacing="0" cellpadding="0" style="background-image:url('https://via.placeholder.com/30x300/09f.png');background-repeat:repeat-x">
This makes me wonder why it didn't work for the author. I have few theories:
a) back then it didn't work
b) it does not work on div elements
c) link was broken
d) missing single quotation mark
e) single and not double quotation mark around attribute style
f) despite the width and height on style, div was of 0x0 size
(Some of those above sound stupid)