Image Only Pop Up is Not Responsive

Having an issue with the pop up not being responsive.

I've added the image in the Text editor and added the CSS mentioned in this thread:
https://premium.wpmudev.org/forums/topic/how-do-i-make-an-image-as-the-entire-popup

Tried tagging width/height at 100% both inline and in the CSS. Still not responsive. Not sure what I'm doing wrong.

I can certainly supply the URL where I'm testing this, but would rather do so privately. Thank you for any prompt help.