I wish to load a progressive gallery from a file

I have a game at the moment with a blank background. I wish to have the background change (cycle) through images in an external folder, selected by the user from their PC at the title screen of the game.

I don't know if this is possible, or if it is how difficult it may (or may not) be. But as I have never used unity before, and in fact have never coded - any help would be appreciated, if not to solve it directly then to help me understand how to approach the task!