You can store your downloads in your account when you signup!
Set your new app on these two iPhone 6 mockups back to back. On the left the black iPhone 6 stands in portrait view and to the right is the white iPhone 6. They both hold an image of 750x1344px. They are set on white backdrop and would be the perfect setting for any new app.
Drag & drop not supported, but you can still upload via this input field: