I just noticed that when a user searches for a search term that would display moderated image results, Google doesn't display the link to search images.

For example, when searching Britney Spears (http://www.google.com/search?q=britney+spears), Google gives links to search News, Video, Images and Music; however when searching for Jenna Jameson(http://www.google.com/search?q=jenna+jameson), only News and Video.

Has this always happened?

Since May 2007. The links to image search and the rest of the specialized search engines are displayed above the Google logo. The links you noticed are dependent to your query and only show the services that are likely to provide useful results for your search.