to edit the image file I'd normally use fireworks (sorry... didn't mean to say firefox before
) or photoshop.
There is a hundred ways and a hundred image editors to change the colour in an image. The easiest way is to use an image editor that your used before and use the paint bucket tool to replace the colour.
or...
you can remove the image from the dialog template file... edit the skin1.css to give the DialogTitle a background colour of that green your using.