Questions on files location and use of tags and categories

Where are the uploaded files physically kept and for image files how might I make them available to plugins like Folder Gallery?
Please also suggest how we might use the tags and categories to organize the pictures into groups (hiking group for example) and that groups activity (for example: a hike at the XYZ park)? So the Hiking Group would want to be able to see the groups activites (i.e. the hikes), select one and be able to get a Folder Gallery of the pictures taken on that Hike.