border around a selected image in preview on the mac- for example a border or line around non- uniform shapes like a hand.

I love preview in what it can do but now I am stumped. Its so easy to put a border around a square, but not a non-uniform shape i.e. hands.

Anyone know of any tricks on how to get a border to conform to the image whether its in preview or photoshop? I know that in preview, you can sketch a line around it but its so tedious especially when you can only control the lines using the trackpad on the mac . There must be an easier way of doing it surely?

Thanks in advance!