How to pick up a random key from a table in Lua?

I'm using this code to draw a random pics from this table

FishImages = {image1 = love.graphics.newImage("bg/fish1.png"),
            image2 = love.graphics.newImage("bg/fish2.png"),
            image3 = love.graphics.newImage("bg/fish3.png"),
            image4 = love.graphics.newImage("bg/fish4.png"),}

with this function love.graphics.draw({FishImages.image1#--I guess the modification is here },pos.x,pos.y)

so how to pick up a random key from a table in Lua ?