Hi,
If you don't get good assistance here, try the
gtk-perl mailing list.
I couldn't reproduce anything that matched your description of the problem.
Instead, I got strange positioning and doubling up of the image - perhaps because the size of the image I was using was different to the one you were using.
If you do contact the gtk-perl mailing list, I suggest attaching your cat.bmp so that they have exactly the same source image as you're using.
And if you like to provide a link to that image, I'll grab it and try it out here .... but it's likely that I won't be able to help you much beyond establishing whether or not "it's the same for me" ;-)
Providing a link to that image might also help others here provide some assistance.
Cheers,
Rob