A clip art picture is a pre-made picture which you can use to illustrate your documents. Most clip art is free to use, but some comes with terms and conditions which restrict how you can use the pictures.
Downloading new clip art pictures
You can install a large collection of clip art on your computer, for use with LibreOffice, OpenOffice.org and other programs. This clip art is produced by the Open Clip Art project.
Install the openclipart package. The package is large, so may take a while to download. (In newer versions, install openclipart-libreoffice and openclipart2-libreoffice.)
To insert clip art in LibreOffice or OpenOffice.org, press Insert -> Picture -> From File. This will browse the gallery directory of OpenOffice.org but in LibreOffice it opens the ~/Pictures folder. (In newer versions, click Tools -> Gallery instead.)
If you don't see the clip art as expected, open a Terminal (Applications -> Accessories -> Terminal) and type the following:
sudo ln -s /usr/share/openclipart/png/ /usr/lib/libreoffice/share/gallery/clipart
sudo ln -s /usr/share/openclipart/png/ /usr/lib/openoffice/basis3.2/share/gallery/clipart
Note that both PNG and SVG versions of the clipart are installed by the openclipart package. In the commands above, /svg/ can be substituted for /png/ for SVG versions of the clipart.
Now you will see several new categories of clipart in the Gallery.
Other sources of clip art
Below is a selection of sites which provide downloadable clip art: