How To Add My Domain before the external image link

How To Add My Domain before the external image link

please help me,
I want to add my domain link before the link image .
Exp :
img src="http://picasa.com/example,jpg
I want to change it to be
img src="http://mydomain.com/http://picasa.com/example,jpg
and I want to apply it on the post content, without going in index .
Thanks