How do i get the imagedata from a loaded img?

if i have:
<img id='myImage' src='generate_image.php' />

and then i want to copy the current picture and send it whit ajax.

Can i do something like:
var image_data = document.getElementById('myImage').data;

(I can't use ajax or read the file, as it generate a new picture each time, i need to copy the current one)