Mrs. E. L. CLARK was photographed in October of 1863 at H. Manger's Photograph Gallery, on Chestnut Street in Philadelpha. It also has the mysterious phrase, "rats & mice" scribbled on the back of the photo.
Update: the notation "rats & mice" may refer to Mrs. Clark's hairstyle. Margaret Mitchell offers up a description of a hairstyle called "Cats, Rats, and Mice": "The hair was parted in the middle and arranged in three rolls of graduating size on each side of the head, the largest, nearest the part, being the 'cat.'" (Gone with the Wind, Chapter XIII)
Our second photo, taken at Groom's Carte de Visite and Photograph Gallery, on Second Street, Philadelphia doesn't include a date, but provides this inscription, "Master D. F. DICKSON. Sab. School Scholar. 1st R. P. Ch, Pa."
Both hairstyles in these photos hint at a 1860s time frame, as does Mrs. Clark's subtly pink-tinted cheeks. As you can see, there's always a few clues hiding in cartes de visites.