Changing Image Alt Text Using Replacement Variables

[dog-tag]

Hey all

Just been trying to change my pesky image alt text using replacements variables.
My current alt text is -

alt="Click image for larger version. 
(I think the lightbox feature creates this.)

Anyway I created a replacement variable like this -

REPLACE : alt="Click image for larger version
WITH : alt="$vboptions['bbtitle']

I also tried {options}, $vboptions['keywords'] and {title}

It will change the alt text but then the text is just {options} etc it won't pull the unique thread title or keywords into the alt text for me.

Can somebody tell me the correct code I need?
G