It's not completely finished, but it works for now. It's just that our mods wanted this feature to be used asap, so... I'll polish it up later.
FOr now, when a pic is wider than 300 px, it will automaticly gets resized to a more appropiate size. It will have a dotted red border and you can click on it to view the full size.
You still needs to download the complete file though, so it's not saving bandwidth. It just helps preventing your screen to be all stretched out.
Here's an example: