I forget which one of the secret pictures is where, so I'll just list both: one is found in the hidden object room at Lady Carhen's house. I believe it's the center candle flame you'll want to click on. The other is found by clicking on the shield in the coat-of-arms on the Stonehold map.
#25 is found by talking to the spymaster after you attend the ball. Just keep clicking the options to talk about how Lady Ballymore liked you.