Userscripts.org

Neat! If you'd like to use it together with scripts like Instant Gallery, you can change it marginally to:

if (location.pathname.indexOf("gallery") > -1) {
  for (var i=0; i < document.links.length; i++) {
    var link = document.links[i];
    if (link.pathname.indexOf("pic/") > -1) {
      link.pathname = link.pathname.replace(/\/[^\/]+$/, "");
      if (!/[^\/]+\.[^\/]+$/.test(link.pathname))
        link.pathname += ".jpg";
    }
  }
}