My selection is the bottle, and I have to choose Selecte--> Inverse, before pressing the delete-key to get rid of everyting but the bottle. You could choose to make a selection of the background instead, and then just press delete after making the selection. Make sure the picture is a layer (not locked) before you press delete - or else you won't have a transparent layer. Make sure you save it as a png file (interlace: none).
