The image that you supplied is one pixel too short.
Here's a little tip for you, if you don't use layers or have not figured out how to create layers in gimp. Now is the time to, layers are your friend.
First keep a layer of the original EQmain.bmp it should look like this:
Then create another layer with your image that you want to use for the background of the skin. Now to create the titlebar's, you can create another layer with the EQmain on it again and place it over top of this image and reduce the opacity to 50% or so. Then you can use it as a guide line to make your skin.
Or if you do not wish to do that, remember that the titlebar on the EQ and Main windows are 14 pixel's high and 275 pixels long.