The Open Clip Art Library is a continually growing archive of user contributed clip art that can be freely used. There is over 7000 items of clip art archived. The Open Clip Art objects are available for download in svg and sometimes png format.
Unfortunately, OpenOffice does not support the svg format but there are a couple of solutions available. Use a batch processing utility called svg2office to convert the svg files to odg files in bulk or open the svg file in Inkscape and re-save it in the odg format.
OpenOffice does, however, support the png format.
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