Be careful if the photo includes a person’s face prominently. If it does, the person in the photo has likeness rights in the photo[0] which the photographer cannot assign to you without a signed model release form from the subject of the photo. Most Flickr photographers are amateurs who don’t have model release forms for their photos.

Plenty of lawsuits have occurred around the use of Flickr photos without model release forms (there is also a major sub-plot in the Kevin Smith movie “Jay and Silent Bob Strike Back” that revolves around likeness rights).

Creative Commons is not enough when there are prominent faces in the photo.

[0]https://en.wikipedia.org/wiki/Personality_rights

Seconding this recommendation. A former hobby of mine was photographically cataloging museum exhibits and uploading them to flickr cc-licensed. I've pulled tons of cc photos from flickr for picture books and other projects. It's a fantastic resource, and only costs you attribution.