Oops! Sorry, @rossmitchell: I didn’t realize you had responded already. I’m leaving my response below for the added explanation.
Your background image is working as intended by the theme’s developer: it’s meant to only cover the middle section of the page(excluding the header and footer). The header area has been designed with its own background colour of white.
If you want the background image to be applied to the header section as well, we need to either unset the header’s background colour, or change it from white to transparent.
The above strategies, translated to CSS code, are as follows (Place either ONE of these these codes in APPEARANCE => CUSTOMIZE => ADDITIONAL CSS):
.site-header {
background-color: unset;
}
Or:
.site-header {
background-color: transparent;
}
Try it out and let’s see what happens.