This is what my resource folder looks like:
As cmd parameter I use: --resources /path/Resources
Since I'm not using the PackageMaker GUI and my installer tool creates the Distribution XML file manually from various bits and pieces, I can only say that you have to specify this background.jpg file so that it ends up in the Distribution XML file as:
<background file="background.jpg" alignment="topleft" scaling="none"/>
Ie despite it being in the en.lproj folder the file should not be specified with a path.