It’s actually not the image that is transparent. There is an overlay box on top of the image (and THAT is transparent). 🙂
There are several ways to fix it.
The clean way
To find the overlay in the backend and see if you can disable it. Or perhaps make it 100% transparent (opacity: 0). Depending on the theme, then this might not be an option, but I would guess that it is (somewhere).
I’m not familiar with that theme, specifically.
The dirty way
You could simply add this to your themes custom styles:
.sydney-hero-area .overlay {display: none !important;}