When you upload your book cover to Atticus, it is normal to see the top and bottom of your image cut off in “Edit Book Details”. Since Atticus is primarily for formatting, it does not mean to display the entire image for your cover.
If you want to double-check your sizing is correct, check out this article here: Calculate Image Size for Print and eBooks. This helps walk you through calculating the right size for your cover for print and ebooks.
For E-Readers, you'll always want to upload the image according to the KDP specs. Atticus may not show the entire image based on your screen resolution, but it will export in its entirety! You can verify this by proofing the final copy of your ePub using Kindle Previewer: https://www.amazon.com/Kindle-Previewer/b?ie=UTF8&node=21381691011
We hope this helps!