From Rossen:

Mixing shape and contour might get a bit too much. We

Suggestion: 
<img id=shape-me url=foo>
<style
  #shape-me {
    shape-outside: contour; //equal to 'shape-outside: url(foo)' shape-outside: attr(src as url);
}</style>

Following discussion with Alan Stearns, we don't think a 'contour'
keyword will be easily understandable here. As smfr pointed out above,
we have several ways of solving the issue

  1. using a second reference to the url through url()
  2. getting the url from the @src attribute through attr(src url)
  3. getting the image itself using an image() notation

Hence closing as WONTFIX.