Not citing the source of photos, or using copyrighted photos, 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 reposting under a Creative Commons license, please attribute and link according to the specific license. If you are reposting under CC0 please consider noting that at the end of your post.
If you are actually the original author, please do reply to let us know!
The text is written by me, but I used a photo from the Internet. Everyone will take a photo from the Internet anyway
Not providing a source leaves other users to believe that the images you post are ones you have created. You'll see many here that do use images from the internet, many that are successful here use public domain photos and/or cite sources where they find the images.
#photography is a tag used by photographers for their original works and discussion of photography techniques and photographers themselves.
Users who feel deceived may flag you.