Nivo-slider arrows with larger clickable area?

Hi folks,

https://www.lociarchitecture.com
Rollover “education” – click on “carroll school library”.

I’m using the nivo-slider plugin for all project images. I want to make the clickable area around the left and right arrows larger, but changing the padding values on them messes up their positioning and reveals that they are embedded in one image.

Here’s the css…
https://www.lociarchitecture.com/wp-content/plugins/nivo-slider/scripts/nivo-slider/nivo-slider.css

Here’s the art…
https://www.lociarchitecture.com/wp-content/plugins/nivo-slider/scripts/nivo-slider/themes/default/arrows.png