You don't copy and paste it into a new document, you copy it and paste it into the alpha channel of your current document. The little window in the bottom right that shows your layers, click on the tab that says channels. You should have an R channel, a G channel and a B channel. Now you need an alpha channel that contains your pic so paste it into an alpha channel and then cover the parts you want to show in white and the transparent (non-shown) part black (think backwards, like me and you'll get it
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