Bug 846306 - Image shouldn't be displayed before loade

Fix for [#846306](https://bugzilla.mozilla.org/show_bug.cgi?id=846306)

At the moment you can see that the image is being loaded in the detail screen. In the bug report I considered two options to fix this, either hide the image until loaded or show the thumbnail and render when done. I've spend some time on the second approach and the problem here is that:

1. Setting the thumb is fine
2. Loading a 4 MB large image on the DOM will flicker the element, thus not making it smooth in the emulator
3. On the Unagi this takes even longer, so it doesn't seem like a viable option

So I've gone with the first approach, the image will only be displayed when loaded completely.