BBcode help

[pein87]

I was working on a new bbcode that would allow my users to make a better list them what vbulletin already has. the code I wrote was:

<li style="list-style-image:{option}.png">{param}</li>

I them mmde its show as this:

[bullet=start]New RPG postbit[/bullet]

I then made another bbcode like this:

<ul>{param}</ul>

and used:

The problem is it doesnt work at all. How would I make it so that this bbcode worked and so they can choose the image of there bullet for the listed item?