Yes. Open the Photos tab on the left and click Add Photos. You can upload from your computer or import from Google Photos. The new photos appear in that panel — drag any of them onto a page, or into an empty photo slot.
The Photos panel holds every photo in the book, whether it came from a contributor or from you. Photos that are not yet on any page show under the Unused filter.
Tap Help at the bottom right of any page and tell us what is happening. Your message comes straight to us and we usually reply within a few hours.