Moving image in circle with touch move

Question

I want to move an image within a circle with my touch on move. I've been confused how to determine the coordinates circle boundaries when the finger moved.

For example I want to move this image like lock screen on android tabs.

http://cdn.alltouchtablet.com/wp-content/uploads/2012/03/samsung-galaxy-tab-7-plus-lock-screen.jpg

EDIT :

Now After learn the answer below, I managed to draw an circle image and moving this image around the screen.

And now, I want to restrict the image movement so that it only can moved in a circle, just like in the image above. Any ideas?

I hope you can help me. Thank you!

Answer 1

so I've actually not done anything like this but i know this tutorial : and the guy does something similar to what you want,

this video is one right in the middle of an entire playlist of tutorial videos if you dont get what he's doing check the previous few... as for moving the image in a circle i'm sure there'll be some way to set boundries :)

http://www.youtube.com/watch?v=9d4Wua-cxZs&list=PLB03EA9545DD188C3&index=32&feature=plpp_video

hope it was helpful!