As a source, I used "Ohio Photographers 1839-1900" by Diane VanSkiver Gagel.I found many photographers in Preble County, but don't know which would have been the one you are looking for.Your family may have gone to Darke or Montgomery Counties since Harrison Twp. is so close to them.
William Walter Appleby - in Camden from 1878-87.
James E. Barhart - in Eaton from 1880-84.
James S. Barnhart - in Eaton in 1878.
James Beatty - in Eaton in 1900.
Bonner & Brown - in Camden in 1890.
Bonner & Ross - in Camden in the 1890's.
Ed Bonner - in Camden in 1900.
Leon M Bostick - in Camden from 1889-1900.
Cassius Brower - in Camden in 1900.
Frank Fisher - in Camden in 1899
James C. Fisher -in Camden in 1899.
Scott Fowler - in Camden in 1895.
Harlan & Carder - in Eaton from 1875-76.
Harlan & Lewellen - in Eaton in 1886.
Calvin C. Harlan - in Eaton 1873-1900.
C. M. Huddleston - in Camden in 1879.
Marion Jenkins - in Israel Twp. of Preble Co. in 1900.
R. M. Jenkins - in College Corner from 1896-97.
E. M. Kennedy - in Camden from 1872-73.
W. J. Lamler - in Eaton from 1888-89.
George Larken - in Camden in 1900.
W. J. Lawler - in Eaton from 1888-89.
J. Calvin Lewellen - in Eaton from 1884-1899.
A. McAdow - in Eaton in 1864; in Camden from 1866-68.
W. O. Miley - in Eaton in 1886.
Horace Morris - in Eaton in 1850.
S. I. Phares - in Camden in 1894.
William Phearis - in Gratis from 1860-65.
Homer Pollock - in Camden from 1894-96.
Irvin R. Reedy - in Eaton from 1894-99.
John E. Rhea - in Eaton from 1864-68.
H. F. Rogers - in West Alexandria from 1860-65.
Albert G. Rohrer - in Camden in 1870.
W. D. Ross - in Camden in 1894.
Fred L. Severance - in Camden in 1900.
St. Clair Picture Gallery - in Eaton from 1893-99.
Minnie Stephenson - in Camden in 1900.
Frank E. Walters - in Eaton in 1900.
William D. Wooley - in Camden from 1860-65.
William Phearis, H. S. Rogers, and William D. Wooley look to be your best bet.There may have been more photographers in Preble County at the time of your photos, but I don't know who they would be.The book I mentioned is an excellent source.The only problem is that the photographers are in alphabetical order and not by location.I had to search through the whole book to find those in Preble County.I know there are several from Darke and Montgomery Counties, but I don't know all of the little towns of those counties to be able to make a complete list.I hope this can be of some help to you.