Not sure what sort of effect you're after, but copper onion domes with virdigris..I came up with this. I started with a basic virdigris turquoise, applied an inner glow to the circumference and used an HSB noise filter to give the dome some texture. Then added some cracks and a rip in it (again using filters). Finally applied a fisheye distortion to it to make it more 'dome like'. Then I just popped on the rectangles and circle to represent the ribbing and finial and gave them a drop shadow.
Feel free to use them if they suit. I popped in a black and white version just in case, but hopefully now you can make your own! The filters I used were all from the Alienskin range. Well worth the money. The main application was paintshop pro. I would not be surprised if the GIMP has comparable filters which are free.