You can store your downloads in your account when you signup!
If you are looking for a billboard mockup, why not use this stage, with the towering billboard against a bright blue sky? You screenshot must be in a 2:1 ratio, to better fit the shape of the ad, but you can also crop it after uploading. Impress your clients with this mockup.
Drag & drop not supported, but you can still upload via this input field: