Under Moral Rights law, you must provide sufficient acknowledgement of the work copied, identifying the creator/author and the work from which the copy is taken (by its title and other descriptive information). Acknowledging the source of your work also helps avoid claims of plagiarism.
Moral Rights include:
A standard attribution statement should include:
If you don't know the source of the work, you need to indicate this with the following statement:
Source unknown. All reasonable efforts have been taken to identify the copyright owner of this material. If you are the copyright owner or know who they are, please contact email@example.com.