Hello I'm not using Openoffice on my Slackware box but I would suggest the following:
- Grab the openoffice.org Slackbuild (for example from here
- modify it at the end just before the "Fix ownership" section with something like this:
tar xvf OpenClipart.tar.gz
cp -a OpenClipart_directory $PKG/opt/openoffice.org3/share/gallery
Basic idea is untar the OpenClipart file and copy the images in the openoffice gallery (or wherever those images are copied when you import them using openoffice GUI).