You can store your downloads in your account when you signup!
For the best app mockup use this black iPad resting between friends on a wooden table to show it off. The iPad is being held up by a girl opposite her friend to show him your app. The iPad mockup is in landscape view. It holds an image of 1024x768px. This is great for a casual or social app.
Drag & drop not supported, but you can still upload via this input field: