How to add thread title into IMG ALT Tag

[jiggle]

Hi,
I am using vBulletin 3.7.x and want to add an img alt tag to all my images. At the moment the postbit_attachmentimage shows

alt=""

I want to add the thread title and the filename of the image.
So I change the postbit_attachmentimage to:

alt="$attachment[filename]"

This works. But I was not able to add the thread title to the ALT Tag. I tried $thread_title, but this does not work.
Is someone able to help please ?
I want to optimize my images for google images and an alt="" is not very good for google :-)

Many thanks and best regards
Andy