Public domain images can be easily separated, even ones for commercial use, by going to the advanced feature in Google. Simply click on the gear thingy in the upper right box on the search landing page, a drop down menu will appear with the advanced option. On the advanced page near the bottom is a drop down menu where you can choose from different rights attributions. Option labeled "free to use, share or modify, even commercially" covers all uses of a picture, meaning images in results exist in the public domain for commercial or non-commercial use, and you can take the puppy dog and perhaps merge it with other public domain images of puppy dogs so your mailing has pictures of puppy dogs all over it.
If you are looking for historic pictures, the Library Of Congress website has thousands of pictures dating back to the nineteen century that are public domain images and free to use. Many of them are from all over the world. These are in the public domain and free to use or modify for any purpose.
Wikipedia also has an enormous database of public domain images in their Wikimedia Commons area, though many of these are under what is called a Creative Commons license. Creative Commons allows the rights owner to assign responsibilities to the user. Typically all this means is that if you want to use a Creative Commons picture then you need to credit the rights owner, perhaps giving him a link to his website on yours. This allows the rights owner to benefit from his work without demanding financial compensation or putting the work freely in the public domain.
See that picture below, that I also a public domain picture.