Bringing in images from other image, result are ragged edge!

Hi

Having one of those days! Hopefully someone can help, I'm sure this is obvious.

I am bringing a cutout image from one picture to use on another picture. But the new one is incredibly ragged at the edges, and I can thnk out why!

The resulotion on the second is half the first, but it can't be that much of a difference. COlour modes the same. Bit stuck.

Thanks for any help.

Mark