Drag an image from the image tray on the left by clicking and holding, then drop the image by releasing the image over the desired image frame. The image frame can be empty or filled. If it's empty the new image will populate inside it. If the image frame already has an image inside, then the previous image will be put back in your image tray and your new image will replace it.
There is also an "Auto Fill" function in Zno Designer that will automatically design your photobook based on the number of photos uploaded and number of pages. If you do not require a specific order to layout your photos, this is a time saving function.