They seem to be included in the epub file. I can copy a cover, but not put it back.
The standard size would probably be in pixels. The covers seem to be jpgs. I had a bunch show up in Gallery on an Acer 100.
If the book was originally digitized to fit an early Kindle or Nook, the screens would probably not be as high res as your Toshiba, and probably not 10"
If you bought the book and illustrations are too small, ask the vendor/publisher.
I found an mp4 of an early video. It looked OK on the lower res 7" Coby, but blurrier on the higher res 7" Acer.