Documentation:
Image Box

Link

Here you can choose whether or not the image should link to a URL or open in lightbox. Options include:

1. None
2. Link to URL
3. Link to Lightbox

When Link to URL or Link to Lightbox are selected you can choose to activate a link overlay.

Image Link URL

If you would like your image to be a link, input your destination URL here.

Image Link Target

Here you can choose whether or not your link opens in a new window.

See here for more details.