jueony said:
Thank you very much! Now we completed Let's Make ★ Character　CGイラストテクニック series!!
You're amazing!

And to remove page margins in calibre

1. [Preferences]-[Common options]-[Page setup]
Select "Default input Profile" from the input profile and set all the margins to 0.0 pt
2. Apply

3. [Preferences]-[output options]-[pdf output].
Check 'Preserve aspect ratio of cover' option.
Set all the page margin to 0.0 pt.
4. Apply

To preserve aspect ratio,

1. Add your kindle to calibre library.
2. Check the size of the cover image.
3. [Convert books]-select 'PDF' from output format-[PDF output]
Un-check 'Use the paper size set in output profile'
On the 'Custom size', Put the exact size of the original cover image(ex> 1480x1920) and select 'devicepixel' from the Unit.

4.Ok

By this method you can remove page margins and you can get the right aspect ratio. In this method, you should always check the size/aspect ratio of the original cover image and set the custom size manually.

So I think it's easier to convert your kindle into 'zip' format(or Kindleunpack to folder), and extract image files from it and make a pdf with the image files by using other software.