Tiger never had a setting for choosing the tooltip contents - it's always just used a default (even Turtle didn't have such a setting). The only option is to choose whether you want the caption to appear below the image thumbnails, or to appear as a tooltip when you hover. That setting is where it's always been: Tiger > Sections > Images > Place thumbnail caption.
So, I'm not sure what you're trying to do. Could you explain a bit further?