Technically, Mr. Pekin, it is not allowed.
Tahai, it is not enough to give credit to rightful owner. You must first receive permission for use, and then give the proper credit.
As for the subject of the image, the creator of image must obtain a model release from the subject, if the copyright owner commercially exploits the image. This is a separate right from the copyright to the image and is known as rights of publicity. In the case of a pet, the owner of the pet provides the release.
I don't know, Iain. I think, if he's using it for educational discussion in a limited environment, not making profit, not claiming it as his, and providing a link back to the original source, that would be considered fair use.
Mr. Pekin Duck, the safest bet though is to always use your own photos, or if you must use another's photo, just provide a link to direct people to the site the photo is on.