Well, on Linux, it's Tk::JPEG, notice the small k. Maybe windows ignores cpaitalization.

If you want to "customize" Tk::Thumbnails, you should read "perldoc Tk::Derived". It shows you how to do it. There is also a good article by Steven Lidie at Custom Widgets . And as always, goggle for "Tk derived widget" and "Tk custom widget", for examples.