Alfred Hitchcock:
in the first moments of the film in the crowd - he is the only one not applauding the speaker. He can also be seen in a following scene, after the body in the river has been found, standing next to a couple discussing Jack the Ripper.
Despite Blaney's repeated claim that Rusk is the murderer, the police apparently don't investigate him or show his photo to Monica until well after Blaney is convicted.
After Blaney escapes from the hospital and is trying to find Rusk, he starts opening his apartment door and opens it about a foot. In the next shot from the inside, we see him opening the door again.
Richard Blaney: [announcing himself to his wife's receptionist]
You can inform Mrs. Blaney that one of her less successful exercises in matrimony has come to see her.
Chief Inspector Oxford: No, discretion is not traditionally the strong suit of the psychopath, dear. Believe me, that's what we're dealing with. You ought to read his wife's divorce petition.
There are several terrific sources for screenshots from Alfred Hitchcock's 1972 film Frenzy. http://www.hitchcockwiki.com/wiki/1000%20Frames%20of%20Frenzy%20%281972%29 is one of the few sites that has a wide variety of high quality, hi-res screen-captures from the various films of Hitchcock.
There is no official site for screenshots.