I like the way you click on the bookmarklet, then click on the image.
Would be nice to have some sort of indicator after clicking the bookmarklet so the user knows that the images have become hot. Just a discrete little flash message, something like "Okay! Now select your image."
// Flash message
var flashMsg = document.createElement("div");
flashMsg.style.position = "absolute";
flashMsg.style.right = "0px";
flashMsg.style.top = "0px";
flashMsg.style.padding = "20px 40px";
flashMsg.style.backgroundColor = "#eee";
flashMsg.style.borderBottom = "1px solid #ccc";
flashMsg.style.borderLeft = "1px solid #ccc";
flashMsg.style.fontSize = "30px";
flashMsg.innerHTML = "<em>Okay!</em> Now select your image";
document.body.appendChild(flashMsg);
window.setTimeout(function(){
flashMsg.style.display = 'none';
}, 3000);