Why is Abslom Daak's profile a drawing and not a photograph? Because no-one's gotten close enough to take one and live.
When the Teller makes his first appearance, the Doctor's team of would-be robbers are right there, yet the creature doesn't sense enough guilt from them to pick them out of the crowd along with the unlucky man who got his forebrain soup-ified. The Doctor attributes this to them all successfully not thinking about their own plans to steal from the Bank, but considering Psi outright complains about how hard that is to do, it's difficult to accept that excuse. It also can't be that the Teller sensed their mission's real purpose at the time, and deliberately let them pass, because it had to dig deep down into the Doctor's mind at the end to discover that they'd come to free its mate. Rather, the reason the Teller didn't sense any intention to rob the Bank from the four of them is because they didn't want to rob the place at all: they wanted to get out of their situation alive, and were playing along with the Architect's plans merely for survival's sake. Wanting to stay alive isn't something people need to feel guilty about, and their wish that they hadn't been roped into robbing the Bank in the first place far outweighed any desire to succeed at the theft.
Which makes the Doctor's little speech about imagining the thing you want most in the world a bit of a Nice Job Breaking It, Hero, because up until then, the Teller might not have detected them in the tunnels so easily! After, Psi and Saibra actually wanted to rob the vault to obtain their respective desires.
Extra Manipulative Bastard points for the Architect, when you realize that the path the Doctor's team took into the tunnels was probably intended to lead them past the cells where the Bank's brain-souped would-be robbers were kept. If the four of them hadn't seen what a nasty fate awaited them if captured, they might have hesitated to use the Shredders and not been safely teleported into orbit.
The fact that Madame Karabraxos assigns her own clones to run her bank branches makes more sense if you consider that the bank's security screening is largely based on DNA identity-verification. Her clones have the same genetic pattern as she does, so by exclusively using such copies as managers she can keep the number of genotypes that can bypass high-level security measures to an absolute minimum of one.
The Teller picks up feelings of guilt. Okay. But how about...
Guilt that's unrelated to bank robbery? E.g. about cheating on your oblivious spouse, or having a screaming match with your child earlier that morning, or an I Did What I Had to Do moment in your Dark and Troubled Past? Or even breaking your diet? How much guilt does it have to be? And is it specific? Just how telepathic the Teller is, exactly? If all it actually picks up is fear (of being caught), doesn't it have lots of false positives? Not necessarily among people who've done heinous things, just those who suffer from anxiety or guilt complex.
Presumably the Teller is discerning enough to distinguish the run-of-the-mill "background level" of guilt that any empath can probably pick up on in a crowd from the more directed, immediate, and active culpability of someone who's about to rob the place. Possibly it might misfire if, say, the enraged spouse of one of the guards were to turn up at the bank with the intention of stabbing the two-timing cheat, but it's doubtful if Ms. Delphox would give a damn about such collateral damage.
And even disregarding that - there still are psychopaths, who don't feel any guilt. Blue-and-Orange Morality adherents and Just Like Robin Hoods may well think robbing a bank is a good thing. They would, provided with the right tools, be able to just waltz in and out, take all valuables and leave a can of alphabet soup, just for the delicious Pun.
Considering how the Teller was diverted by Psi's data-binge of other criminals' police records, it seems like it focuses more on self-awareness of felonious intent - i.e. "guilt" in the legal sense - rather than just "feeling guilty". After all, Psi wouldn't and couldn't have felt legitimately guilty over other peoples' crimes: if anything, he was feeling proud of his Heroic Sacrifice on another's behalf.
