If you choose to import a single layer graphic, you may want to embed an alpha channel. An alpha channel contains the transparency of a document saved as the fourth channel in an RGB image. Here's the fastest way to create an alpha channel in Photoshop for a single layer document.
A perfect alpha channel starts with an active selection loaded. To load a layer, Cmd+click (Ctrl+click) on the layer's icon in the Layers palette until you see the marching ants . NOTE: If you have a layer style applied, you must flatten the layer style by merging it with a new empty layer.
Switch to the Channels palette and click on the Save Selection as Channel icon or go to Select>Save Selection>Operation New Channel.
Choose File>Save to save your native .psd file. Then pick File>Save As, and choose to save a PICT or TARGA file. Make sure the Alpha Channels box is selected.