Thoughts, a bit complex perhaps:

  add a netbsd-logo package that puts logos in /usr/pkgsrc/share

  add a similar pkgsrc-logo package

  add a os-logo.mk file that depends on the right package, perhaps
  defaulting to the NetBSD one if a specific OS version isn't available.

  make pkgs like thttpd use os-logo.mk and also depend on pkgsrc-logo.

-- 
        Greg Troxel <gdt@ir.bbn.com>