I know very good option with Phoca gallery, plugin and editor buttom for Images-gallery insert. Very automatic and easy to explain to a beginner.

They have "Switch images" images show, so check if you can style it as you wish.

Otherways i know plenty of plugins that would do that you want. But it is not something for a beginner. They need to use FTP client, make folders, copy code in K2 Item.