Using image as the lightbox button

By default, the Lightbox shortcode shows as a blue button with text, which opens the lightbox content, but I need my own image instead of a button.

The problem is that when I enter the <img src=”…”> where the button text would normally be, the images does indeed show as a button, but with the blue button around it.

So how do I remove the button css altogether from the lightbox shortocde so that the only thing showing on the screen is the image (with no blue background), that when clicked on opens the lightbox?

Thanks,
Rob

https://wordpress.org/plugins/shortcodes-ultimate/