You can store your downloads in your account when you signup!
A teenager wearing glasses looks at his iPhone 6 mockup with your app in while working at his desktop PC with your website of choice in it. This excellent iPhone 6 mockup and desktop PC mockup is designed to compare a mobile version and a desktop version of a design. If you are looking to show these differences, what better than to show it in context?
Drag & drop not supported, but you can still upload via this input field: