Lord of the reads: +5454
This question has been edited and transferred from a longer question here
In the site cover creator, I found that the spine area provided did not match the file at all. I imported the spine file, but only a fraction showed. I did save the file at the dimensions stated, and at 300 DPI. My (wild) guess is, the site for some reason has the spine option geared for uploads at far less than 300, thus when a file is loaded at 300, it goes hogwild and doesn’t constrain itself in the given area. I know this is guesswork by the way, but the upshot was, my correctly sized spine, dimension wise, didn’t fit even though it should have done.
The Cover Creator is geared up to accept images at 300dpi. If you have already created your front, spine and back cover as whole images then the best thing to do is upload them to the cover as a background image.
You can then adjust the cropping area to ensure that the image fits correctly by clicking on the ‘Adjust’ button next to the upload button on the left hand side of the cover creator. It takes you to a place where you can drag and set the cropping area. If the image is large, it may be that you need to click on adjust and extend the area of the image that displays.
Look for this in the cover creator:

Hope that helps!