What is Public domain and is there exceptions to the rule

CC0 1.0 Universal (CC0 1.0) – Public Domain Dedication

The person who associated a work with this deed has dedicated the work to the public domain by waiving all of his or her rights to the work worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.

You can copy, modify, distribute and perform the work, even for commercial purposes, all without asking permission. a link to us Phototeria.com is appreciated. See Other Information below.

There is some Exceptions :

Identifiable people may not appear in a bad light, unless they give their consent. identifiable people must give their consent for public usage of their images. This is named Model Release. I would personally not use images with people on it.
Do not suggest endorsement of products,…. as example: do not use an image of Air Force and place it to your own product as if if they have given there ok.
The same goes for private property which is then named as Property Release. The object owner must give permission to use pictures. Property Release also covers public buildings and designs of new computer or TV. If you want to use images like that, the owners must be asked for permission.

To this topic there is also a “but” . Model- and Property Releases are particularly important for commercial use. If you are showing an image on your blog, it is non-commercial, editorial usage. In general, that should be ok. Commercial use is loosely defined as all sorts of businesses, where you are actually selling something, or if you use images for advertising purposes.

Now its not that complicated as you think :

Place yourself in the Image owner position owner or designer: Would you not want to be asked first? That is the question you should always ask yourself before using a public domain image without release.