Let's look at the ways in which you can upload an image from your computer or from the web, straight into your book.
First, click the + button and then choose Import. You'll see a toolbar with three options:
Choose a file from your computer
Clicking on the blue button will open up the File manager on your computer. You can navigate to the image/video you want and choose it by double-clicking.
Connect to Drive
Likewise, you can choose to Connect to your Google Drive account and access any media you have saved there. You'll need to log in after you click the Connect to Drive button.
Once an image or video is chosen, it will be added to the page. You may find it takes a while to load, depending on the size of the file and your internet connection speed.