With no copyright, the monkey’s image is in the public domain.
When a work is in the public domain it is not copyrighted. Works in the public domain are free for anyone to use however they like. But even when work is in the public domain, it’s important to cite it so your reader knows where it came from.
Copyright protections don’t last forever. When a work’s copyright expires, it becomes public domain.
|Date of creation||Copyright expires and work enters the public domain|
|Before 1923||Already expired. These works are already in the public domain.|
|Between 1923 and 1978||95 years from the creation date. On January 1st, 2019, works created in 1923 entered the public domain.|
|After 1978||The length of the creator's life, plus 70 years|
|Works for which the creator was paid (i.e. works for hire) regardless of creation date||The person or organization paying for the creation owns the copyright for 120 years from the creation date. After 120 years, the copyright returns to the creator or their estate.|
2019 was a big year for the public domain. For the first time in many years, copyrights began to expire, meaning that more literature, art, music, films, etc. can be used without permission or payment.
For example, in 1923 the novelty song "Yes, We Have No Bananas" was first published in sheet music form. On January 1, 2019, this song entered the public domain, and it can be recorded without permission from its copyright holder. However, recordings of this song that were made since it was originally written may be still be under copyright. For example, in 1950 bandleader and singer Louis Prima released his recording of the song. Louis Prima's recording will enter the public domain 95 year's from it's creation date--2045.
For more information on how these rules came about, start with this article from the Atlantic Monthly.