The system only supports "one at a time" uploads for now because that's the most compatible way. When you try to allow multiple uploads simultaneously you run into all kinds of trouble. You also run the risk of breaking the system for inferior browsers like Internet Explorer which people still use. Everytime I make an advanced feature, IE people get left behind. I don't want to exclude them from such an important feature such as uploading photos.
One day we'll support uploading more than one at a time, but that will certainly only happen after the vast majority of people no longer use IE.
