It is really a problem with the ebook, not with the ereader. It might be a conversion problem, or if the ebook is originally ePub then it is just publisher incompetence.
In some cases you can fix the too-big images by adding this CSS to the book's stylesheet, or to the Extra CSS section of Calibre's conversion settings if you are converting from another format::
Code:
img { max-width:100%; max-height:100%; }
If the problem is just with the cover image then you can use Calibre's "Polish books" feature to replace the cover.
But in other cases you might need to dig around in the book's CSS or html to figure out what the publisher has done wrong before you can fix it.