It's not an image. The texture is constant, but the shading is dynamic. Try resizing a window horizontally (not vertically), and you'll see that the texture of existing parts of the window doesn't change; only the shading does. The texture is never scaled at all.
So the appearance of every brushed-metal window is dynamic which is why they're so god-forsakenly slow on older hardware (uuuurg!).
The texture itself is probably a small repeating pattern. Dunno where it's stored, but I don't think it would be of any use to you, anyway. Interface builder is the way to go. Just remember that scaling the image you capture will not give you an accurate representation of a real metal window.