Karl,
I think you'll have to buy a Mac after all. I found this
http://perso.wanadoo.fr/billbobuilder/program to build masked and cropped billboards for Artlantis 4.5.. The hitch is it's Mac-only!
I also found this link to free billboards, trees etc: Very nice ones (but French ;-):
http://www.archicado.com/download/download.htmlBoth linnks were found on the Italian English-speaking Archiradar page, which is becoming very useful:
http://www.archiradar.com/eng/
