Not citing the source of photos(images) is plagiarism. Here is a post on why this is bad, and here is how easy it is to find allowed photos.
Creative Commons: If you are posting content under a Creative Commons license, please attribute and link according to the specific license. If you are posting content under CC0 or Public Domain please consider noting that at the end of your post.
Not indicating that the content you copy/paste is not your original work could be seen as plagiarism.
If you are actually the original author, please do reply to let us know!