You can store your downloads in your account when you signup!
Is your app aimed at bikers, athletes or is it a GPS app? Then maybe you can use this Samsung Galaxy S4 mockup. In this stage, a biker holds his Samsung Galaxy S4 and looks at it intently. You can drop an image here that is 1080×1920 pixels.
Drag & drop not supported, but you can still upload via this input field: