It is absolutely great to come across sites that offer free stock images. Cost wise, silhouette is much cheaper than buying stock images and or vector based illustrations. If you used silhouette in a right way when designing, it can make your design look way cool and stand out. Here are some of the few sites that I have come across that might be helpful in using them in your designs and or presentations.
http://www.123rf.com/search.php?word=silhouette&&searchopts=&itemsperpage=60 (this is another stock site image I came across that is also reasonably priced.)