If by "mail header image", you mean the standard layout at the top of the email that is defined by the template upon which it is based, you may be able to change text that is in it using the Email Designer for each email you create. It all depends on whether the text in the template has been defined as editable. You can tell if it is editable by the appearance of a dashed box around the editable text when you move you mouse pointer over it. To edit it, just double click on it.
Building on what Elliott said, you can achieve the effect you are looking for if you can create an image that blends into a solid color. This way you can create a table row with a matching background color in which you can place your <div> tag to edit text. The finished product looks like one piece but it is in fact two. Hope this helps. Feel free to ask any questions.
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