First, click the + button and then choose Import. You'll see a toolbar like this:
Clicking on the Files tab will open 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 the Google Drive tab and access any media you have saved there. You'll need to log in after you click the 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.
Other things to note with Google Drive
Google Drive is a 3rd party integration and doesn't always show all of the files.
You can try searching a file by name and see if it can find it.
Alternatively you can create a new folder and see if that displays when you look for it. If it displays then I would suggest putting all the files you/your students need in that folder.
Also see: How do I revoke access to Google Drive?
Book Creator for iPad
In Book Creator for iPad, all of your photos and videos are accessed via your iPad's Photo Library. So when you tap on the Photos in the menu you'll see a list of your albums from which to choose a photo or video. Just tap on the one you want and it will appear on the page.
Another way of accessing photos and video on your iPad is by tapping the + button, then the More tab and then choosing Files. This will access the Files app which is built in to iPadOS.
If you want to add photos or videos from the Files app on your iPad, which will give you access to 3rd party drives, read our article: Share content to Book Creator for iPad from your cloud drives.
Once you've uploaded a photo or a video, you can read the next steps for: