How Do I Specify An Element To Use With 'Pop Up' to Only Show When Clicked?

Hi guys, I'm trying to add a 'pop up' to only show when a certain image is clicked.

I'm struggling to get the pop up to show. I thought I could do something like this in the 'Element Selector' field with the 'On Click' rule added:

a title="30 Day Guarantee"

...But that doesn't work. Any ideas how I can show a pop up only when a certain image is clicked?

Thanks
Rob.