You can store your downloads in your account when you signup!
This wonderful multi-device mockup shows an iPhone 5s mockup and a Nexus 7 mockup next to each other. The iPhone is in portrait mode while the Nexus is in landscape mode. With this device mockup, you can showcase two different versions of the same design. One for iOS and one for Android.
Drag & drop not supported, but you can still upload via this input field: