I found it15 Jun 2006 The matrix is [a b c d e f] where (a,b) is the X-vector, (c,d) is the Y-vector, and (e,f) is the position vector.
After some playing around I managed to get an image in my postscript file.
But just one ...
After more experimenting, it seemed that "colorimage" was not able to render the image twice in 1 page.
So after a long time of searching, I decided to re-implement the colorimage function and write my own version. It draws every pixel as a box of the correct color.
Using that, I was able to put logos on all our labels. The printer took 3 or 4 minutes to render the postscript though (66x 40x40 pixels images)