Most articles on Wikipedia can benefit from an appropriate illustration. To find an image (or a video or sound file), try browsing related Wikipedia articles as well as doing some searches on Wikimedia Commons. If you have an original image you created, you can upload that to Wikimedia Commons and then add it to Wikipedia articles.
The basic code for adding an image to a Wikipedia article is like this:
[[File:Example.jpg | thumb | This is the caption.]]
This video walks through the process of uploading a photo and adding it to an article. For more advanced image syntax, check out the picture tutorial.