The first rule of the Google Search Engine Ranking Algorithms is You Do Not Talk About The Google Search Engine Ranking Algorithms. Google ranks pages based on a combination of multiple proprietary algorithms working in tandem based on multiple criteria at any given time. There is no single result that every person receives. Individual search results are tailored geographically, and even personally. If you are logged into Google, your Google account information affects your search results. Even if you aren't logged in, your recent search history and browser history affect your search results. The more a web designer tries to game the algorithms by adding multiple key words to her page ("stuffing"), the more Google tries to penalize her for gaming the algorithms.
The best advice (or so Google asserts) is to concentrate on the goals of your company and your audience, rather than on the search engine rankings. We don't have a company, and the goal of our hijack is explicitly related to the search engine rankings. So ours is a unique conceptual situation.
Fortunately, Google Images operates according to different algorithms and criteria than the front page of Google. For instance, Google Images provides a sampling of images, because no one wants to see duplicate results of overly similar image on the same page. Also, Google Images tries to ignore logo watermarks. Here is a very instructive video explaining the basics of the Google Image Search algorithms: