Such a fun one. I have not done a very deep dive but a quick search pointed me to Frog Rock in Eastford, CT, a glacial boulder painted to resemble a frog in 1881 by state legislator Thomas J. Thurber, who thought its natural shape resembled a squatting frog. This frog-painted rock does NOT appear to be that one, but suggests a late 19th/early 20th century trend toward painting boulders to resemble frogs, perhaps especially common in Connecticut! And with a clever hand-painted sign hung up the hill from this one, lending this particular rock-frog, at least as I read it, the identity of a doctor or fortune teller or advice giver (maybe specifically a therapist)-- as in "rock in" and open for business! Maybe that's just me, but the frog's wide open mouth certainly advances the reading!  Postmarked Branchport, CT 1908.  

5 1/2" x 3 1/2". Good condition, real photo postcard, stamped and postmarked.