Adding class="thickbox" to Image Assist Filter-Tag

Hello!

Can you help me please? I'm searching for a solution where I can type:
[img_assist|nid=|title=|desc=|link=url|url=picture.jpg|align=|width=|height=|class=thickbox]

That means that I want that the filter-tag automatically gets the class "thickbox" added.
Is there a solution via the input formats?

Thanks in advance!
Thomas