Most of them are CC BY licensed, the only thing mandatory for those is appropriate attribution (some images might be licensed differently, see individual image in Flickr which license applies). Usually you’ll find a suitable credit line on the image itself or in the image description on Flickr. In case you have questions regarding an image, you can reach me via email or you can direct message me on Twitter. For some of the images I can provide unannotated versions for publication, if needed.
While not mandatory, I would appreciate you sending me a short note, telling me where you have used the image.