eomalia on "Passing variables through links?"

On my website, http://www.tremarctos.com, I have several photo galleries sorted by category. Clicking on a photo sends one to an individual photo post, and one can navigate between individual photos using arrow buttons. I'd like the site to remember which gallery the photo was accessed from, and for the arrow buttons to go to the next photo in the gallery (not the global post order). I don't want this to change the permalink, but to be treated as a temporary variable. Is there a good way to do this? I also wouldn't want to do anything that would preclude the use of caching plugin.

For example, if you go to the "mammals" gallery and click on a photo, I'd like the arrow keys to navigate through the mammal category. (If a photo is accessed directly by link, the arrow buttons should navigate through all photos)

Sorry if this is well-known; I am a beginner.