Within your main thumbnail processing loop try creating a image object, doing the thumbnail, then explicitly clean up by calling $image->destroy() and undef $image.
Sure, it will be significantly slower (you make a fresh instance, use it, and dispose of it each time), but you should not leak memory. Update: This worked for me with a very similar problem using ImageMagic, but from the above reply it seems there is an intrinsic fault with destroy() in Tk.