path problems with mongrel and themes and filecolumn

Hi there,
I'm running a themed rails app and have just switched to mongrel which I have running proxied from apache2 on ubuntu.
I have the app up and running and it works both via apache and calling the mongrel port direct, but I have a problem from within the theme when running a validation check on images controlled by filecolumn.

I have file column images in:
/public/images/artwork/...

theme template in:
/themes/name/views/artworks/show.rhtml

with in the template I have:
<% for @image in @artwork.images.select{ |img| img.valid?() and img != @artwork.preview_image } -%>

which gives me (briefly - stack trace below):
ActionView::TemplateError (No such file or directory - images/artwork/78/lying_in_newspapers.jpg) on line #60 of themes/hayleynewman/views/artworks/show.rhtml:

I assume it's now looking the theme root rather than rails public root, but I have no idea how to deal with it.

This worked fine until I switched to mongrel. Does anyone know of something I need to tell mongrel to stop this happening?

thanks
dorian