JavaScript can change HTML attribute values.
In this case JavaScript changes the value of the src (source) attribute of an image.
<'button'onclick="document.getElementById('myImage').src='pic_bulbon.gif'">Turn on the light
img id="myImage" src="![pic_bulboff.gif](https://cdn.steemitimages.com/DQmUTHYT5xBY4bQXchTcmjXEtS6YR6CxsAP3uwnaYrhSLky" style="width:100px">
<'button' onclick="document.getElementById('myImage').src='pic_bulboff.gif'">Turn off the light
