Thanks.
Yes, you can remove the image from settings/media but every time my client adds a new image, it repopulates and he can't remember to do this manually every time.
I tried the code, but while this removed the image, in place it leaves a black background which creates the same effect where the fonts in navigation aren't visible. I want it to remain white (which is what happens when I remove the image from the media). In other words, the same effect as if I had removed the image manually. When I simple hide it, the result is a black navigation banner.