Unless you are the original artist, not citing the source of images might be flagged for plagiarism. Here is a post on why this is bad, and here is how easy it is to find allowed photos.

Not indicating that the image you post is not your original work could be seen as plagiarism, especially if you claim or imply to be the original artist.

Unless you clearly credit all media sources, you run the risk of being flagged and/or blacklisted by a growing number of anti-spam entities on Steemit.