Bug 1071514 - [Messages] The picture will flicker once when you take one picture as attachment r=schung

* don't rerender the resized image in the iframe, instead keep displaying the
  unresized image. This will take slightly more memory but hopefully not too
  much.
* still updates the file size in the same iframe
* insert iframe's content using a blob URL
* tweak some iframe style so that it's fully loaded asap