[NTG-context] images as background

Hi all, for a presentation, I would like to have a series of images as backgrounds for my slides, and I want to use them in a randomized order. The images are named "1.jpg" through "20.jpg." So I was thinking of using this code: \externalfigure[\directlua0{print(math.random(20))}][width=\paperwidth] epecting that this should But that doesn't work, ConTeXt doesn't find my pictures (I get "name: dummy" texts). Any idea what is going wrong? Thomas