You can store your downloads in your account when you signup!
Place your screenshot for your apps on this extra large, mockup. The black laptop sits on top of a beige tabletop. Behind the stage is a white wall with a window. The small view outside the window shows greenery, as in a garden. The stage itself is a closeup of the laptop at a right angle. A huge 1366 x 768 px stage displays your product over 80% of this image.
Drag & drop not supported, but you can still upload via this input field: