Replace URLs automatically with function.php command?

How can i replace URLs automatically with a filter/actionhook in the functions.php?

Reason: I use the “HTTPS (SSL)” plugin to use shared ssl.
It should look like: https://any-shared-ssl.com/www.mydomain.com/somePicture.jpg

But it only works for the navigation links. When it comes to the entry content, the links look like this: https://any-shared-ssl.com/somePicture.jpg
That’s right: “/www.mydomain.com/” always dissapears.

How would the entry in the functions.php look like to make this work?
🙂