Ehh, I think I made a workaround with ptf2rco, but I've posted a number of different versions of it, I don't know which one you got.
This one is my latest version, so ya... http://endlessparadigm.com/upload/ptf2rco.rar

Note that this work around allows you to push it through Z33's tool so that it decompresses, though RCO Editor won't really be able to identify it.  When you do open the decompressed RCO, select the wallpaper, and extract as a GIM image, then rename it to a .bmp file.
Hope that helps.