Create an array of images, then select the image randomly:
images = {
display.newImage("MyImage.png"),
display.newIMage("MyImage2.png"),
...
display.newIMage("MyImage5.png")
}
randomImage = images[math.random(1,6)]
@krs added that you can also use a table of file names and instantiate only one of them:
local imageFiles = {"MyImage.png","MyImage2.png", ... "MyImage5.png"}
local imageFile = imageFiles[math.random(#images)]
randomImage = display.newImage(imageFile)
If you chagne the image often the first technique is better since you only instantiate once, but it depends on your use case.