How do I create a medal system based on the score

I want to display images of medals on the high score screen as achievements. For example, if the best score is higher than 25 but less than 50, displaying a silver medal png. The main objective for this is to create achievement board within the game. My score.lua is based on http://coronalabs.com/blog/2013/12/10/tutorial-howtosavescores/ <---- the document provided by corona. Can anyone help me out?