Remove mouseover text on NextGEN nav buttons

I’m using the NextGEN shutter 2 navigation.

When I hover over the navigation buttons, there is this usual yellow text box showing “next”, “previous” and “back”.

I know it shows the name of the button picture file.

But how can I delete or change this text in NextGEN?

I can find the “link” in Firebug, bur I can’t locate the file to change it.

Here is a link to the page. You’ll find the navigation buttons I talk about when you click on a thumbnail: https://www.kjellgunnaroveroye.no/