Create Kindle Cover with Mobipocket Creator or use the first page of file?
That's a question of endless debate, but here are some Kindle screen shots to help you decide which is best for you.
The images below, scanned from my Kindle's display screen, compare the two methods.
The cover created "by default" (below) because it is the first page of the MS Word file, has margins all the way around, and includes locations and other information that is on other pages of the file.
To get the full page image shown in the Mobipocket Creator sample, the image must be a "true" 600x800 pixels.
A smaller image "resized" or "stretched" to 600x800 pixels won't work.