How to cut out a bit of an image?

Ok, lets say you had a pic of a blue circle on a white background. And you just want the blue ball in a .png, on the old apple works you could draw a scribbly line around something like a blue ball on a white background and it would snap to fit around the blue ball so you could move the ball anywhere on the page.

How can I do this but just save the blue ball as a png so there in nothing around it?