Syntax for extensions

[Florian Lindner]

Hello,
I need to to slightly extend the markdown syntax. (place an image (img tag) 
in text which URL has not yet been determined). Therefore I want to define 
something like $[Alt text](img.jpg) which would be replaced by my 
pre-processor with ![Alt text](/path/to/img.jpg) and then sent to markdown.

Is there any special syntax that should be used for such 3rd party extensions? 
How would you do it?

Thanks,

Florian