You can store your downloads in your account when you signup!
Set your website mockup on an iPad sitting in its keyboard deck with an iPhone 5s gray nearby. The iPhone 5s will hold an image of 640x1136px and the iPad will will hold an image of 1024x768px. Both are sitting on a wooden desk with a notepad an pen nearby. You get the perfect stage for a side by side view of your app and website.
Drag & drop not supported, but you can still upload via this input field: