You have to form a legal javascript string in this line to get the .src
property assigned as you wanted. So change this:
document.getElementById("image").src = images / GreenLight.jpg;
to this:
document.getElementById("image").src = "images/GreenLight.jpg";
FYI, the error console would probably have been your friend here as this probably would have been a javascript error and would have given you the error and line number.
Your HTML should also use quotes:
<img id="image" src="images/GreenLight.jpg" style="visibility:hidden" />
And, if you want a 5 second delay, then you need to set the time to 5000 milliseconds:
setTimeout(function () { showImage('image') }, 5000);