These Currier and Ives Clip Art images (or other media files) are in the public domain because its copyright has expired. This applies to the United States, where Works published prior to 1978 were copyright protected for a maximum of 75 years. See Circular 1 "COPYRIGHT BASICS" PDF. Works published before 1923 are now in the public domain.
These Currier and Ives Clip Art images are also in the public domain in countries that figure copyright from the date of death of the artist (post mortem auctoris), and that most commonly runs for a period of 50 to 70 years from the last day of that year.