I don't know how long the credit may be, but maybe something like this: "Photographs on pages X, X, X by Rama Rama. Released under License X. Distributed by Wikimedia Commons: http://commons.wikimedia.org"
I have used Commons images on three occasions in a book, an online magazine and on a postcard. I credited with the photographers name if it was there (not UserName) and Wikimedia CC license # but I did not use the website, I will next time. I left a note for permission on User page but no one replied. Wayne Ray