Diamond Shape Cut Out...

This one might be easy to some but i cant do it.....

I have some pictures that I want to cut out some diamond shapes from 
them.... what i'm aiming to do is make some personalised signs for the
back 
window of my car, the sort of thing you often see, such as baby on board 
etc...

I want to lift a diamond shaped section from an image, then over type some

text i.e. Pillock on board....... ideally, i'd like to put a coloured
border 
round the diamond shaped image...

Can any kind person give me a clue as to how to cut a diamond shape and
then 
put a border to it.

Many Thanks

Dave :)