You can store your downloads in your account when you signup!
This iPhone 5s is ready for an app mockup. The iPhone is set on a dark green tray. There is also bowl set on the tray as if it was just set down at a cafeteria table. This setting is perfect for an app for students. You can fit a 640x1136px portrait view image onto the iPhone.
Drag & drop not supported, but you can still upload via this input field: