Well, I didn't use anything as the trigger. I used whatever Powerstore v2 used as default. Like i said, I didn't change anything but the button image.
But yeah, perhaps Powerstore did use it as the trigger? Anyhow - What I did was make the search button an image, without any text, and than just used CSS to have the submit input use the image and formated the text to fit over the image. It works now, although it just seemed strange I had to go through all that just to use an image for the search submit.
I'd probably do it different next time, but for this project, what I've done is working now and it good enough for me.