You can store your downloads in your account when you signup!
You can use this to get a great comparison shot of an iPhone 5s and a Samsung Galaxy side by side tilted slightly towards the left on a dark wooden table. The iPhone 5s will an image of 640 by 1136 pixels and the Galaxy will hold an image of 1080 by 1920 pixels. This is perfect to show your app mockup in two different formats and would work well for just about any sort of app.
Drag & drop not supported, but you can still upload via this input field: