O.K., so I'm confused... the link you give shows you how, right? Although the leaf in the pic is part of the original. So, if you want something from a different image, let's say a pizza.... it's a similar process... only you'd have to do a similar selection on the other pic of the pizza... once selected you do a copy. Then go back to you work image and paste as new layer... you've already greyscaled your work image... put the pizza where you want it. For the overlaps, like the little fingers... you'd have to do a similar function from the original image to bring in that section as a new layer... once you get everything close to nicely lined up you'd maybe want to flatten the image and touch up any minor flaws with a clone brush or tool of your choice...
O.K., enough .... far too late to really think this much. ;-)