Click on the layer visibility on the layer named background. You'll get a red cross showing as in the pic but you wont notice any difference to the picture.
Hit the delete button and all of the white will disappear leaving you with a chequered background.
Now choose the eraser tool. and very carefully rub out all the bits you dont want. If you accidentally rub out a bit too much just go to edit then click undo. You can undo as many steps as you want.
There are quicker ways to get rid of the waste areas but that's for those who have more of a knowledge of the selection & drawing tools. I will try to do a tut on it but not for a few days at least ;o)