Support for background-cover is spotty at best, but in this particular case, there is a known rewrite where Gmail will alter this rule to be background-cover: contain. It's usually for the best to size the image to cover the correct amount of space and just use media queries if you're looking to have it scale dynamically.
I normally try to stay away from background images in email all-together, but when I do use them, I'll usually add them to a table or div instead of the <td>. In this case, you might try nesting a div inside that <td> and adding the background properties to that? Another thing that I noticed is that you've got a bunch of background-ish things set -- background-image, background, background=, background-color, and bgcolor=. You might try re-arranging these, I've seen mixed results when using the attribute (background="___") versus the inline style (background:____;), you might try using just the background-image property on a table or div. You'll still run into issue with Outlook and a few other clients that dont support bg-images. I also agree with Courtney that it's best to have the image sized appropriately in the first place to help hedge issues like this.