This error was quite hard to track down, and was also the cause for gnome-web-photo timing out. Things worked when running from the shell, but when running it through our web service (as another user), gnome-web-photo went misbehaving!
Reports on the intarwebs suggested changing ownership of the .dbus directory (so dbus can work properly), but as we don’t have a regular home directory for the user (we actually have to set a fake one to fool mozilla when it’s creating its .mozilla directory) we didn’t have any .dbus directories.
After toying around and getting nowhere fast, I finally had a bit of luck with just running dbus-launch before gnome-web-photo. This worked once, then things went back to being crap. After reading through the dbus-launch man page I finally found out that you can make dbus-launch set up the environment for proper dbus access and then launch a command line you provide.
dbus-launch /usr/bin/gnome-web-photo -t 30 --mode=photo --format png ..