Changing the Graphics of an Object with Lua?

I wanted to know how I would go about changing graphics with Lua, if it's even possible. I'm relatively new to Lua, and wanted to play around with graphic things. An example of what I'm trying to do (not what I'm trying to do) is change the crawla graphics based on character, or change the player graphics depending on whether or not a custom button is pressed. Are things like this possible?