[ltk-user] Problems with photo-image

Hello: I have a problem with images on my cbr viewer. Supose I load a image on a photo-image instance called photo:

(image-load photo "photo.ppm")

If now I load a second image which is smaller than photo.ppm, with

(image-load photo "photo2.ppm")

It loads photo2.ppm at first plane, but I seed photo.ppm at the zone that's not filled with photo2.ppm. At the moment, I'm loading a image "black.ppm" that is a just a black image to reset the background, but I don't want to do It. There's some function to reset the photo-image instance's background before loading another image?

Thanks you.