I thought this tutorial on adding clip art to OpenOffice galleries was very helpful on a feature that I know my wife and I have used regularly and had to figure out on our own by the road of hard knocks. Solveig spoke at our open source user group a year or so ago and was a great resource for advanced uses. She obviously had a lot of professional real world experience not only with OpenOffice, but also it’s proprietary counterpart to be able to compare and contrast them. If you use OpenOffice, you might want to follow her blog as she has lots of great tips and tutorials.
You can also check out the Open Clip Art Library for more free and open source clip art. Feel free to help by contributing your own art as well. If you have Ubuntu, it’s available as a package from the repositories for easy install.