[Python-markdown-discuss] Adding a new tag - completely confused

I'm trying to make a few additions to the markdown syntax to be able
to do a few things I like to do but I'm completely confused of how to
get it to work.

An example: I want to be able to use an image tag like

@![Hello](imgcode)

and get something like

<figure><img src="http://stuff.com/imgcode/blabla.jpg"
alt="Hello"><figcaption>Hello</figcaption></figure>

>From what I understand this should be handled as an inlinepattern and
if I look in the source it's basically the ImagePattern that I need to
copy and make a few changes to. So I did that and made a few cosmetic
changes to test that I got things working ... but I have no idea how
to actually get Markdown to recognise my code.

I looked at a few extensions, read the docs but I still don't get how
I can use it. What I've done is simply to copy the image pattern and
the ImagePattern class and changed the pattern to

MYIMAGE_LINK_RE = r'@\!' + BRK + r'\s*\((<.*?>|([^")]+"[^"]*"|[^\)]*))\)'

and ImagePattern to output "srct" instead of "src". But how do I get it to work?