Yes—if you make and share a copy of a CC BY-NC image, attribution is still generally expected, even if you modified it and even if the use is personal and non-commercial. A spoken credit alone is not ideal, since it can be forgotten.

For a print or photo book, a practical approach is to include written attribution in a place that suits the format, such as:

• a credits page at the end of the book
• the book flap
• the back of the print
• a caption, if appropriate

A simple credit naming the photographer and pointing to the original source is a sensible way to respect the license. For example: credit the creator, mention that your image is based on their original photo, and include a link or reference to the Flickr page.

If you want to be safest—legally and practically—contact the photographer/rightsholder and ask how they’d like to be credited. They may approve a simple credits-page note or offer different permission terms.