Where to get Content, Images and Media
Of course, the top place to get content is from your users. But remember the 90-9-1 rule. Only 9% of your users will contribute.
You can get data from spreadsheets, dictionaries, and 3rd party data suppliers. See this example of how I have loaded massive amounts of content into people's wiki websites.
Free Images and Media
You can obtain free images from many sources, including MediaWiki commons, which now has over five million files! Of course, you can always create and post your own content and media. Just be aware that the content you supply to the wiki will follow the copyright rules set forth by the wiki. Just follow the rules set forth by the owner.
Paid Images and Media
The best is to use your own images, and use yourself or your customers or employees as models. For a great treatment of this, visit http://www.webinknow.com/2009/10/who-the-hell-are-these-people.html
You can buy and use images and media (video and audio) from third party sources such as istockphoto.com.
If you use images or media such as video or audio, use your own material first, material you bought, (from stock photo websites, for example), or include it in a published photo stream from Flickr or some other service.
Be careful using images. One of the first websites I designed, I used an image from another source, with no malicious intent or understanding of copyright rules. I was hit with a bill from the image copyright holder in excess of $3,000.00.
You can get good data from your website from public or private databases. See my article on Structured_and_unstructured_data for more information.
Many sites create content "on demand". An example of this is
For a full treatment of this concept, check out this article from the November 2009 Wired Magazine