To delete an image from your Image Gallery, first click the image icon in the content toolbar (as though you were going to place a new image in your book, even though you won't be in this case), and then click the green 'My Image Gallery' button.
Once your gallery pulls up, you can then click the three little vertical dots next to the name of the image you wish to delete, and select 'Delete from Gallery.” You’ll see a pop-up confirming whether you want to delete the image, and you’ll just need to click Yes.
Once you've deleted all the images you'd like to delete, you can just exit out of that image popup box and back to your book project.