Well there's a few different ways...
Nice and simple. Email a picture to yourself, open Mail on your iPad, tap on the picture and choose 'Save Image'. The image will now be in your Camera Roll and available to use in Book Creator.
A little warning
iTunes will optimise your images for the iPad during transfer. They will be converted to JPEGs and resized to be no bigger than the iPad screen. If your images have transparency or are very large (to enable good quality zooming in iBooks) you may want to transfer using one of the other methods.
From the web
If your images are already on a website then you can probably save them to your iPad direct from the website. Visit the webpage using Safari, then long tap on an image to bring up a menu with a 'Save Image' option. Once saved the image will be in your Camera Roll and available to use in Book Creator.
Dropbox (or another cloud service) is the easiest way to transfer lots of images at once from a PC or Mac to your iPad. You'll need to install the free Dropbox app onto your iPad, and upload your files from your PC or Mac to your Dropbox account.
If you have a Mac running OSX Yosemite and an iPad with iOS 8+, you can simply AirDrop the photo from your computer to your iPad.
Enabling access to your photos and camera
When you first try and import photos into Book Creator, or add a photo by taking it with your iPad camera, the app asks for permission to access your Camera Roll or Camera.
Sometimes this setting is changed, or someone accidentally doesn't allow permission, and it needs to be reset.
To do this, go to your iPad Settings > Privacy > Camera (or Photos) and make sure Book Creator is toggled on to give access.
From the blog: