I don’t think you have an image on half the page, what there is is a slider. If you make it full height, even if that’s possible, the slides will be distorted. If you add a background image where there is currently a white background it will obscure the messages. Also large background images slow down page load which is a turn-off for visitors.
But if you must, the css will be something like this:
#content {background-image:url(“… path to your image …”)};
“Feel free to look around” comes from a text slider. This will be in your settings somewhere. I’m not familiar with your theme so I can’t say exactly where.