Generally speaking, RH can only create the proper relative
path to images and other files (htm, pdf, etc.) if the files are in
the same folder structure in the output as they were in the source
when you created the links.
We maintain a 42-project merged WebHelp online help with
thousands of cross-project links; if we are about to create one or
more links to another project, we copy that project to our local
machine (in the same directory structure: all source child projects
under the /projects folder, and all output child projects under the
/mergedProjects folder).
We also maintain two major versions of the software help, and
usually experience link problems when we import topics from one
version to the other (RH gets confused about just exactly where the
links should point to in this new directory structure you've placed
it into). Other than that, it works great for us.
Good luck,
Leon