I observed the problem in one of my own books, a basic Excel manual that I published earlier this year. The images, which were intended to have full-screen widths, were reduced to about 50% of the page width. This made details impossible to read. Of course, it's possible to "blow up" the image by tapping on them, but that ruins the reading flow and also takes time (there are about 100 screenshots in this guide).
I don't have Nook HD screenshots to share, but I'll try to illustrate the problem with some facts and a sample image from the book that was made unreadable. The Nook HD has 1440 x 900 pixel resolution with 243 pixels per inch. This means an image that's half the width of the screen (450px) will be 1.85 inches wide. Here's what a 1.85 inch wide image from my book looks like in a browser (96 ppi):
Do you see the problem?
The file format used by the Nook is an ebook publishing standard known as ePub. Apple's iOS devices use the same ePub files as the Nook. When I was working on Excel Basics In 30 Minutes, I tested the ePub file extensively in the e-reader that I own (an iPad2) as well as in the Nook simulator provided by Barnes & Noble's Pubit service. It looked great in the iPad. On the Nook HD simulator, the images were actually too large for the screen -- Pubit actually warns authors that images may appear strange. I assumed that they would be resized to fit the Book HD screen. Little did I know that they would be reduced to the point of being unreadable.
Any other authors experiencing this problem with their Nook titles?