Javascript : get <img> src and set as variable?

If the img below is present

<img id="youtubeimg" src="http://i1.ytimg.com/vi/VK4ah66jBvE/0.jpg"/>

and the script is

<script>
var youtubeimgsrc = "something here"

document.write(''+youtubeimgsrc+'')
</script>

and the result should be http://i1.ytimg.com/vi/VK4ah66jBvE/0.jpg

what can i do to get the image source and set as a variable.