Black and white photo negative, 3.5 inches by 1.5 inches.
The second photo above is the results of my tinkering in Photoshop for 20 minutes. I don't have one of those fancy negative scanners, so I did the best I could. You can barely make it out, but there are two babies in a bed with the mother (?) looking over them.
Found in "The Genuine Works of Flavius Josephus" published by Mack, Andrus and Co., Ithaca, 1846.