Instagram isn't just for pictures of sunsets and rich kids partying. The popular smartphone photo app has unleashed an army of amateur street photographers on the world. Daniel Arnold was one of the best, producing a stream of brilliantly voeyuristic snapshots of New York City streets—that is, he was until he got banned for posting boobs.

Instagram has changed photography for its over 50 millions of users, and not necessarily for the better. A lot of Instagram photography suffers from an insipid romanticism that is built into the very software in the form of those cheesy vintage filters. There is a serious mobile photo scene—check out the Mobile Photo Group for some of the best—but even they often succumb to the sort of feel-good vignettes that dominate Instagram's "popular" page and attract big brands.

But Arnold's photos of New York daily life are framed squarely in the unvarnished present: His best are funny, creepy and sad in ways that agitate the combined anxieties over lack of privacy and empathy in the social networking age. A few of Arnold's more mischievous shots border on cruelty, using the poor or malformed as the butt of a joke. (One picture of a sweatpants-clad woman with an impossibly huge ass in profile does this literally.)

"The privacy thing is a huge, huge draw to me, the whole phenomenon of privacy happening in public," he said. He sees his photos as "cracks in reality where you can see people folding into themselves and having these really personal experiences in full view of this sweaty, horrible world"—and the world's Instagram followers.

The 32-year-old Arnold, who works for a website he'd rather not disclose, joined Instagram in August of 2011 and quickly developed an obsession.

"I felt like I was on this mission," he said. At the height of his mania, he was snapping over 3,000 photos a week to post just a handful on Instagram. When we met at a bar a few days ago his iPhone was filled with dozens of photos taken in the few minutes he'd been waiting on the subway platform to get there.

Unsurprisingly, the subway is a target-rich environment for Arnold. Like Walker Evans a half a century before, Arnold has perfected covertly capturing the otherworldly weariness unique to the New York City subway rider. My favorites are the super-close-ups, taken inches from faces that fill and overflow the frame. They're so intimate I had first assumed their subject must have been aware of Arnold's iPhone. But at the bar he demonstrated how a skillfully wielded iPhone is perhaps even more discreet than Walker Evans shooting his Contax through his buttonhole.

He hopped up from our table and stood next to me as if I was a subway rider, casually holding his iPhone horizontally in his hand by his waist. This put the lens inches from my face, but it looked like he was just taking a break from Angry Birds. Arnold blindly snaps pictures with the volume button from this position—an ability added with a recent update to the iPhone's operating system.

Arnold has never had a huge follower count—just around 1,500 at his peak popularity. But his admirers were rabid and included many professional photographers and fashion and media insider.

"I love the guy," wrote New York photographer Christelle De Castro in an email. "At the core he's a journalist, which is why he's such a die hard instagram'r. His work is about observation—examining things already happening / stories unfolding, and framing it."

The feedback from this audience played a huge role in pushing Arnold to post increasingly audacious photos, but he didn't realize the importance of likes and comments in shaping his work until they were gone. On June 26, he took a picture of a couple of topless female sunbathers at Brooklyn's Fort Tilden beach. One giggling woman is snapping a picture of her friend with her iPhone—maybe for her Instagram?—unaware that Arnold was snapping them both a few yards away. It's a perfect summer-in-Brooklyn moment, made slightly unsettling by Arnold's peeping.

The photo went viral on Instagram; Arnold's phone was bombarded by so many notifications on his way back from the beach the battery died. But the attention backfired. When Arnold loaded up Instagram after recharging ithis phone he found his account had been shuttered after someone had flagged the image.

It's a lesson in the limits of self-expression on social networks: Instagram's terms of service ban nudity. Arnold had been flagged before, and I wouldn't be surprised if Instagram has begun cracking down on questionable content since being bought by notoriously prude Facebook for $1 billion in April.

A "Free Daniel Arnold" movement sprung up on Instagram and Twitter. Arnold used a personal connection to lobby Instagram's head community manager to reinstate his account, to no avail. (Luckily, he had backed up his photos.)

Now Arnold has a new account, but it's not the same. It's private and has only a fraction of the followers. And the photos have been dialed down with the audience. One of Arnold's longtime followers told me Arnold's recent photos are more self-consciously "artsy," less brazen. Arnold admitted he's hesitant after the ban, which left him feeling as vulnerable as one of his subjects.

"It made me feel like some kind of deviant," he said. "It exposed what a goofy thing it was that I've been doing. You're sitting here with you're stupid fucking cell phone all day, taking pictures of strangers, and thinking you're some kind of artist."

Still, Arnold is some kind of artist. Arnold's new account is at arnold_daniel. Maybe he'll let you follow him, if you're lucky.

Click here to see a gallery of Arnold's photos.