Rail Centres -Brighton, by B.K. Cooper (Ian Allen ISBN 0 7110 1155 9) has some interesting photos of the branch - but no mention of Hartington Road Halt I'm afraid.
Good luck with eventually finding that picture - if ever you do, please PM it to me! If only there had been video cameras round on the last day of normal passenger services in 1932, when my Dad had a ride on it - I've still got his ticket!