There is a good discussion of content analysis online, provided by Colorado State University. It discusses both the ideas behind content analysis, and the practicalities of doing content analysis. Although it doesn't address analysing visual images directly, it is a full and clear discussion of the method.
The full versions of the Global Media Monitoring Project reports discuss their methods and their findings in greater depth. The Global Media Research Center at Southern Illinois University Carbondale has a collection of audios about the media on iTunesU. You can access iTunesU via the iTunes Store – it's on the main menu list.