Background image not filling fluid box

First time responsive project developer here. 🙂 I've been watching Paul Wilson's tutorials on YouTube and learning so much. I'm following the steps he took to add a background to the parent fluid box for a master slide. When he does it, the image takes up the entire slide. When I do it, I get a white "buffer"/border around the left and right sides and bottom. Any ideas on how to fill the screen with the image? TIA!