As long as you do not claim them to be yours, it is on the internet and it is fine. You are not saying they are YOUR photos, nor are you claiming the dog to be somebody they are not. Obviously, the only thing is if it is copyrighted, but again, it is just on the internet so it is iffy. Obviously if asked, it needs to be removed.
That said a lot of people do it without much thought and think if they can find it on Google Images it's okay to use.
Of course there's always:
stock.xchng - the leading free stock photography site