[Pharo-users] How does one specify a format for an ImageMorph based on a binary stream

I am trying to do some simple image manipulation. Basically, I want to subtract one image from another, to see what's left. As an experiment, did aStream := ReadStream on: image1 imageExport. newImageMorph := ImageMorph fromStream:aStream. Unfortunately, this fails because there is no subclass to handle the binary stream. I couldn't find a way to specify what format the image was in. Does anyone know how to do this? Cheers Andy