Not all email clients support background images. You might want to put a default color to appear for clients that don't have a background image.
I typically don't do background images but had a recent campaign and was running across the same issue. Here's some sample code that might help:
<table class="width310" width="626" border="0" cellspacing="0" cellpadding="0" height="180px" style="background: #0077b5 url(imageURL) no-repeat left bottom; background-size:cover;">