Well, yes, your are correct. But, if you think about what the cache feature does, then it actualy has no use on details page, simply because - how many times a visitor will visit same image?
The cache feature saves compilled pages into files and use them next time same visitor opened that page again, instead of compilling the page again. To avoid any security breach, the cached pages are unique for each visitor, that means that one visitor will not see somebody's else cached pages...(guests are exeption)
Because home and categories pages are most intensive (perfomance wise) for the server, they are cached, but details page does not require much server resources, and is much more dynamic then any other pages (each image has its own data to be displayed), that's why its pointless cache it.