i'd just received a reply to this by dalko - thanks very much, my friend!
he thinks that the man in question might be huddie ledbetter, judging by the fact that it's a coloured artist playing a 12-string guitar. that's quite an interesting offer, i'd say, as this may well be the case.
there's only one thing puzzling me again: if it's actually leadbelly (who's one of the all-time greats of folk blues) - why then can't this very picture (him sitting down with his guitar and that door on his right side) be found ANYWHERE on the internet? at least i couldn't trace it down.
on the other hand there are not too many rather well-known 12-string axemen from that era. blind willie mctell might be another contender, but i think i can rule him out as pics of him are few and far between - and this one can't be found either.
anyway - for the moment being, dalko's solution seems to be the most obvious. so, any other suggestions, anyone . . . ?