I'm not sure what you are saying here, and think you may be misunderstanding me about images.
While there could be someone who uploads an image for the test, I don't believe that would be common, since I am pretty sure all of the graphic design stuff happens in house.
However, the person writes the test questions and answers, or the person who approves what the author wrote, would need to see the image, thus it would be downloaded.
For example, let's say there is a math question about a triangle, and along with the test question and answer, there is an image of a right triangle with the length of one side and the two acute angles also noted in the image.
The fear is that the autor or lead who approves things, might be doing so from school or at home, and if an unauthorized person got a hold of the cached webpages/images, then that image or a right triangle might clue them in on how the test is written, or on what a possible question might be.
It could be something even more specific, like a custom map of the U.S. showing certain features (e.g. former territory borders, natural resources, etc.) that could be used on several tests, and again, by having a copy f that image, people could use it to cheat or at least be really prepared for the tests.
I know you have to let viewers of the webpage download the image to view it, but the fear is that the image persists on the user's computer long after they have accessed the image.
For example, an author has permissions on the server side to view a question while it is being authored, but as soon as the lead approves the question and answer as "valid", then that author - who is usually a hired consultant - will never have access to the question again, because for what they are being paid to do, they have no need to go back and see it.
So is Jane Doe writes a test question and answer with an image to support things, then after her test question/answer/image are approved, she shouldn't be seeing it on her computer, which could be at work, at school - if she is a teacher by day - or at home with possibly tons of kids!!
If the content of the test question or answer, or the image get stored on ANYONE'S computer, whether through webpages being stored in temporary locations, or image caching, or something the browser does to temporarily store things, or something the operating system does to temporary store things, then that Test Question/Answer/Image have in essence been "breached"...
Hope that makes sense.
Not sure how that changes your response one the image part?!