I've had good experience with Image::Magick in terms of interface and the ability to DWIW. I'm not too sure if Gtk::Gdk::ImlibImage is the best way to do image manipulation since that's not it's primary function (which is loading images, and obtaining X-Windows drawables^[1]) and it's not terribly portable. As for finding the the 'picture quality' all images are internally stored in 24-bit pixel data in RGBRGBRGB format, so the quality really only matters when you're outputting the image.
HTH

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broquaint

^[1] taken from the docs, along with the the other extract in italics