Brody hands his gun to Agnes so that she can keep Marlowe in check. And then he takes out a second gun. Blackmailers always have to be well-armed—it's a job requirement.
When Brody opens the door, Carmen pushes her way through, holding a small black revolver. Our gun count is up to three now. Yikes.
Carmen has come to retrieve her photos, claiming that she saw Brody kill Geiger. This is a big fat lie, but now the tables are turned as the blackmailer is being blackmailed.
With Brody caught off guard, Marlowe grabs the gun from Agnes.
A scuffle for the gun ensues: Agnes tries to get her gun back, but Marlowe hits her on the head. A shot goes off between Brody and Carmen, and somehow in all the mess, Marlowe ends up with all three guns. (If we're ever in a gunfight, we want Marlowe on our side.)
Marlowe makes Brody hand over all the pictures and negatives. Carmen is making a strange hissing sound between her teeth, and there is froth at the corners of her mouth.
She coyly asks Marlowe for the photos, but he tells her that he'll hang onto them for now and makes her go home.