In the standard aero.msstyles the image you're after is 3px wide x 31px high. Using ResHack or Restorator '07 the image is #891.
If you are using any other/modified theme the number could well be different. If you are using a modified theme, if you like you can zip it up and attach to a post and I'll find the number in there for you.
Because you are modifying a system theme file, you may find you'll need to patch the three files needed to enable the running of non-system themes. Download DeepXW's Universal Theme Patcher and run it before you start with ResHack.
Be aware the image file you are working with is a PNG file!!