When you're dealing with HTML emails, this kind of thing isn't possible. Some email clients will allow you to do background-image and position, but clients like Outlook don't allow it.
The best way to do it so it's compatible with ALL email clients would be to create the composite in photoshop and then slice it up. Then put the images into a table layout, making sure to add the "alt" tags so people can see what the links are. And also make a multi-part email.
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