PHP GD2: image linked to another image

Friends,

I need help on this CODE.
Platform, PHP GD2 Library.

Problem:

There will be 2 image files (one real, one virtual-php) and general HTML file.

http://imagehost.com/abc.jpg (real image file)
http://mydomain.com/abc.jpg (this is a virtual image file, php code within it)

in the HTML -
Code:

<img src="http://mydomain.com/abc.jpg">

Now I want, whenever it gets the mydomain.com/abc.jpg file, it will redirect itself to http://imagehost.com/abc.jpg using the internal PHP code of abc.jpg

In short, mydomain.com/abc.jpg will work as a short-cut to imagehost.com/abc.jpg

How to do that using GD2 library functions ??

NOTE: I have tried mod_rewrite, but then the list is getting longer and longer, impossible to handle