the only thing the coffin keys do is an easter egg to find vincent. nohting special. i think some coffins have items in them. nothing else noteworthy.

the keyholes are for a miniquest in the mansion. its an optional thing. i dont even recall the reward, but its nothing you cant already live without.

and as for the picture question, i think this is how it goes. go downstairs and wait for the guy to walk upstairs. folow him up and wait outside until he walks into teh room. follow him and and you see him switching out the paintings.

i could be wrong, but i think thats how its done.