As @Shannan notes you can think of the task as adding a background-image to text, not the other way 'round. And its use is not llimited to tables; for example, a simple A tag can have
style="background: transparent url(http://example.com/img/background.png) center center no-repeat;"
etc.
I strongly disagree with @Jim C that burning text onto images is a best practice. Creating manageable templates is key, and hard-coding text onto every image is hardly manageable -- and of course defeats any attempt at dynamic signatures.