Hi Carol, I used Blurb's BookSmart download, and all my images were sized to 300dpi (dots per inch - not pixels per inch) and the book photography came back fine.
Some post processing software automatically reopens and saves files to 72 and 96 especially when working with jpeg when each save can reduce the files byte size.
I prepare all my 'printer book' images to 300dpi and check dpi by right click properties (on the image file icon) saving them into a single desk top folder / library file for each seperate book - each image file is given the same single letter prefix for the book, and then simply dragged into the Booksmart screen with no problem resizing in the template guides in its pages.
Having the images saved in a specific folder file name, is handy if you decide to adjust an image (when you see the proof copy)because their printer settings are different from your printer settings.