Apple’s guidelines for ePubs tell us to only use inline images. That is, images that act like text elements in terms of formatting. However, I discovered you can violate this rule in one place, the first page, the page that becomes the cover image. You can use a floating image and stretch it to fill the margins so you don’t get a white border. Be sure to check the box to use the first page as the book cover image. Ignore warnings Pages generates once the file is converted to ePub. (A cover size of 600px x 800px works well for iBooks and other book distributors, though other sizes are possible).
In T4L the other day we talked about the guidelines for creating ePubs for through pages. This link has these and more! Great resource on ePubs. See highlighted section