imagefetcher.user.js

var links = document.getElementsByTagName('a');
for (var i=0;i<links.length;i++) {
    if (! links[i].href.match(/\.(png|gif|png)$/) ) {
        continue;
    }
    var img = document.createElement('img');
    img.src = links[i].href;
    links[i].innerHTML = '';
    links[i].appendChild(img);
}