Adding a background image to quick reply out box

[chick]

I searched using different terms but could not find what I am looking for. I would like to tile an image that surrounds the quick reply box seen in the image below. Could someone help and tell me how to do this please? I put red X's in the area I'm referring to.

[Shadab]

That area is controlled by the .panel CSS class.

(And wrapped in an element with a .panelsurround class)