Edge Animate CC :: Image Swapping With Correct Dimensions?

Hovering over a row of thumbnails, I swap the full size photo following this code :
 
var photo = new Image();
photo.src = 'images/'+fileName; // parameter of the function
console.log(fileName+' L= '+photo.width+' H= '+photo.height);
sym.$('toile').attr({
width: photo.width,
height: photo.height,
src: photo.src
});
 
The full size photo swaps, but retaining the dimensions of the first one (initially on the Edge stage).
 
Even more puzzling, as you can see in the following screen capture, sometimes I can get the correct photo.width and photo.height (even if it isn't correctly assigned to the 'toile' element), and sometimes those values are zero ! And I feel so lost that this seems random.

May be it has to do with loading delay, asking the width and height information before it is available ? Should I necessarily preload the images ?