I don't know how much this applies to windows, but in Linux the Gimp icons appear to be in <${whatever}/share/icons/hicolor/${NxN}/apps/gimp.png, where:
- ${whatever}: some directory in the Gimp installation tree for you (possibly C:\Program files\GIMP2\share...)
- ${NxN}: the icon size: 16x16, 24x24, 32x32, 48x48, ...