I copied and pasted the code from a different example
Here it is:
Markdidj: thanks for the reply.
<img id="myimage" onclick="changeimage()"
width="100" height="180" />
<p>Click to turn on/off the light</p>
Yes, I've understood it was an 'onclick "toggle"' but I still can't explain (in plain English...) why the function is formulated that way. I know it works, but I don't know how.