Where is the image file that makes up the progress bar?

[maxsideburn]

I want to make a custom progress bar with more contrasting stripes, but I cannot locate where the original progress bar image is in the system.

this is what I'm talking about:
http://commons.wikimedia.org/wiki/File:Progress_bar_gnome_ubuntu.png

[mcduck]

As far as I know that's not an image (at least with most GTK theme engines, the pixbuf engine uses images). Instead it's created by the code in the theme engine..