Are you using the webui to upgrade? If you are, I've read the following inside the m0n0wall image from scratch guide.
http://doc.m0n0.ch/dev/image-guide.htmlRename the image as per standard m0n0wall conventions to make sure that the webGUI firmware upgrade works (which insists that the filename starts with the platform name followed by a dash):
I just noticed you wanted to specify an image name prior to build time. Guess the info I provided would be useless to you.