You can store your downloads in your account when you signup!
This is a dual stage. On one side you can see a Google Nexus 7, next to it, an iPhone 5S. They both lay at a comfy terrace table. For this iPhone 5S or Google Nexus 7 mock-up, you can drop two images or choose two URLs and compare them between devices.
Drag & drop not supported, but you can still upload via this input field: