When you convert to epub, you need to make sure the conversion resizes the image for the nook's screen (600 x 730). IIRC, the nook will not attempt to resize an image in an ePub document, so if it is too big it may just not display anything at all.
Alternatively you can convert to PDF instead, then set text size to small, and it will shrink the image to fit the screen, though this may lead to unreadable text.
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