This is how to stop image resizing in Word 2007 - not sure if its the same in 2010
Click the Office button on the top left hand corner | Word Options | Advanced | Image Size and Quality for "Do not compress images in file"
Highlight the Picture > Under the Picture Tools format tab > check the Compression Settings in Adjust section.
But I don't think that's the issue - the issue is why is the image shrunk in Kindle Preview
If you using Windows, try the Mobipocket Reader ==>>
Mobipocket Software to get a 2nd opinion, I've never 'trusted' that Kindle Preview s/w for some reason.
BR