Dynamic Background based on field url
You can do this but its probably more of a jquery problem than a Drupal one since you're changing the background of the page based on an asynchronous web call. Rough idea of how this would look:


attach an onsubmit handler to your form that would load the video from the url
add your logic to find the artist from this url (?) and then find the appropriate image 
then modify the body background using this image, ex:

$('body').css('background-image', 'url(' + imageUrl + ')');