Here are various websites where you'll find various GIMP screenshots :
http://developer.gimp.org/screenshots.html : Developer screenshots, hot new features of future GIMP.
http://www.lynucs.org/?gimp : User screenshot archive, real-life usage of GIMP in mixed desktops
http://mmmaybe.gimp.org/screenshots/ : See GIMP running on OS2, Windows and Macintosh
http://www.gimp.org/the_gimp_screenshots.html : Official screenshots on the GIMP homepage
http://www.gimp.org/~tml/gimp/win32/screenshots.html : 4 screenshots of GIMP 1.x on the Microsoft Windows platform.
http://www.xs4all.nl/~collin/gimp/pr/gimp123pr.html : 3 concrete uses of GIMP 1.2.x for Windows against a neutral backdrop (in a press release)
http://bugzilla.gnome.org/show_bug.cgi?id=141951 : 5 proposed screenshots for GIMP's GUADEC 2004 presentation and for www.gimp.org; concrete uses of GIMP 2.0.x on the Microsoft Windows platform