From http://alfonsoml.blogspot.com/search/label/SimpleUploads Resize image before upload

upload_resized.js

function scaleImage(ev)
{
 var data = ev.data,
  editor = ev.editor;

 var img = data.image;

 // maximum size allowed for images
 var maxX = 400;
 var maxY = 300;
 var imgX = img.width;
 var imgY = img.height;

 if (imgX == 0 || imgY == 0)
 {
  alert("Ops, the image doesn't seem to be valid");
  ev.cancel();
  return;
 }

 // if it's smaller, get out
 if (imgX <= maxX && imgY <= maxY)
  return;

 var ratio = imgX / imgY;
 if ((maxX / maxY) > ratio)
  maxX = Math.round(maxY * (ratio));
 else
  maxY = Math.round(maxX / (ratio));

 var canvas = document.createElement("canvas");
 canvas.width = maxX;
 canvas.height = maxY;

 var ctx = canvas.getContext('2d');
 ctx.drawImage(img, 0, 0, maxX, maxY);

 if ( /\.jpe?g$/.test(data.name))
 {
  // You could adjust here the quality of jpg
  evData.file = canvas.toDataURL("image/jpeg", 0.9);
 }
 else
  data.file = canvas.toDataURL("image/png");
}