You can store your downloads in your account when you signup!
In this Android mockup / Samsung Galaxy S5 mockup, we have a girl smiling the high heel boots she is about to buy and her Samsung S5 mockup with your app in it. She holds the phone in portrait mode, allowing you to drop an image of 1080×1900 px to get started. You can also input a URL and we'll show the website on the phone's screen.
Drag & drop not supported, but you can still upload via this input field: