> `:file' property is hardly could be used for this, because it is used in
> many different places beside svg, so I come up with next solution,
> introducing `:base-uri' image property to explicitly set base_uri for
> embedding images into svg

Thank you very much! I've attempted getting this working before, but
gave up on it. See also <https://debbugs.gnu.org/cgi/bugreport.cgi?bug=19373>
for the code that ended up in the current version of image.c.