You can store your downloads in your account when you signup!
For a perfect social app you can use this iPhone 6 mockup that is being held up by a man hanging out with his buddy. The man is holding the white iPhone 6 in landscape view. His friend is sitting to the right. The iPhone 6 mockup holds an image of 750x1334px. This is a great place for a social or casual app.
Drag & drop not supported, but you can still upload via this input field: