The code toggles between two images in response to mouse clicks on the images or clicking a button. The document's image array is used to accessing the target image. The array allows lookup by name. An anchor/link is used to create a clickable image. The anchor would usually load a URL, but the code changes the action of the hyperlink. Also note, the button uses the same function call as clicking the image.

<a href="#" onClick="clickOnOff(imageName); return false;">

and the button

<input type="button" value="Click Me" onclick="clickOnOff(imageName);">

Notice: the left image changes size slightly. This is fixed on the right by specifying image size in the image tag.

  

Clicking the button will change the left image.
Clicking either image will change that image. 
    var powon = new Image();
    powon.src = 'images/zap.gif';
    var powoff = new Image();
    powoff.src = 'images/pow.gif';

    function clickOnOff(imageName) {
        switch (document.images[imageName].src == powoff.src) {
        case true:
            document.images[imageName].src = powon.src;
            break;
        case false:
            document.images[imageName].src = powoff.src;
            break;
        }
    }