I was having a discussion with a user who included an image in their post without indicating the source. My understanding is that any content that is not the work of the author needs to be attributed, independent of whether the content is copyrighted, or whether its use is permitted by its creator or by "fair use" laws.
I pointed the user to https://travel.stackexchange.com/help/referencing which says:
How to reference material written by others
Plagiarism - posting the work of others with no indication that it is not your own - is frowned on by our community, and may result in your answer being down-voted or deleted.
When you find a useful resource that can help answer a question (from another site or in an answer on Meta Stack Exchange) make sure you do all of the following:
Provide a link to the original page or answer
Quote only the relevant portion
Provide the name of the original author
Do not copy the complete text of external sources; instead, use their words and ideas to support your own. And always give proper credit to the author and site where you found the text, including a direct link to it.
However, the user's opinion was that these rules only apply to text, and not to images or other non-text content.
It would be helpful to have clarification on this point.