Copyright Metadata
The metadata fields asserting authorship, copyright status and usage terms for an image.
In depth
Copyright exists from the moment of creation, but proving and enforcing it is far easier when the file states it. Deliberately removing or altering this metadata is itself unlawful in many jurisdictions, and images circulating without it are treated as orphan works, which weakens the creator's position considerably.
Examples in practice
A file carrying a copyright notice and a licensing web address gives an infringer no plausible claim of ignorance.