Every time I saw pictures of criminals on TV I noticed they all looked alike. Every single one of them. This study has vindicated my personal experience. People cannot tell what kind of criminal they are looking at, but they can tell that the person they are looking at is a criminal of some kind. The folks who did the study even went out of their way to find pictures of criminals who looked the least criminal, to stack the deck against the hypothesis that people can tell the difference by looks alone, and still they got positive results.
The one outstanding issue, however, is that women, but not men, have trouble spotting rapists. The authors suggest this is because rapists have to look as non-threatening as possible in order to succeed in their criminal acts. They admit that this explanation is purely speculative and more study is needed (more $$$ + job security). You can read the full text of the study here (PDF).