Module Configuration > General Settings > Stylesheet > Select the "Use Custom Dynamic Forms Style Sheet" radio button
In the very first class:
.DynamicForms_Maintable {
background:url(images/watermark.png) 100px top no-repeat #f2f2f2;
}
Note: I have uploaded the image to a folder I created called "images"
The format for cross-browser compatibility is: url(your-watermark-path.png) | x-pos(px, "left", "right", blank) | y-position (px, "top", "bottom", blank) | repeat (repeat-x, repeat-y, no-repeat, blank-tile) | color (web color name, hex value)
If you find the image is not showing up, try the direct path using the whole site url.
Also, if you want it snug to the top, don't use the Header, use a text/html form item instead!
You can also use this class to control the width of the entire form, when you have a site constrained to a specific size or want to put a form in a column, etc.