I have an ancient copy of the Rubaiyat of Omar Khayyam (book doesn't even have a date or copyright page).

It was published in 1913. The illustrations are by Edmund Sullivan, who died in 1933, so the copyright on them has indeed expired. I would recommend that you convert them to PNG for better compression. Since they are black-and-white line drawings, converting them en masse from the existing GIF will be lossless and not sacrifice any quality. 0