Omitting photos when backing up an iphone to a macbook with itunes
I am assuming you are not using iCloud Photo Library, but are importing the photos into Photos through a USB cable.

What you can do is remove the photos from the iPhone after importing them, and then sync them to the iPhone through iTunes. As far as I know, when not using iCloud Photo Library, an iTunes backup of an iPhone will include a copy of all the photos in the Camera Roll of the iPhone, but it will not include another copy of all the photos that were synced onto the iPhone through iTunes (Apple Support states that “An iTunes backup doesn't include: [...] Content synced from iTunes, like imported MP3s or CDs, videos, books, and photos”). This way you can still view the photos on your iPhone without having two copies of them on your Mac.