If you use someone else's photo, make sure they have offered their work for use under a Creative Commons license. One of the above photos, the duck eggs, is copyright to me and I have never made this image available for use under a Creative Commons license. Whoever cropped out the copyright clearly visible in this photo:

knew the image was copyright. And linking back to the source does not constitute fair use for a copyright photo where the photographer has reserved his or her rights to usage.