Vinh’s company Lascaux had already started working on a Mixel iPhone app and that soon became the sole focus of the company, which announced in August that it was ending support for the iPad app. Less than a week later, Mixel quietly pushed out an iPhone app that was much less complicated and allowed users to automatically create collages and share them from their phone. While the iPad app was more involved and allowed people to cut out images from pictures, manually place and size images and remix other collages, the iPhone app creates simpler collages with less work.

Even without a big press push or marketing campaign, Mixel’s iPhone app has gotten more than 10 times as many downloads as the iPad app. And it’s been featured twice by the Apple App Store, recognition that the iPad app never received. Khoi declined to share download numbers and said Mixel is still not a viral hit a la Instagram. But it’s now on much stronger footing. Here’s some of the lessons he shared with me:

Creative friction kills: The iPad app was impressive to use but it required a level of creative commitment that scared off everyone but the most dedicated users. The iPhone app fixes that by easily allowing people to import pictures and automatically designing a collage for them. Users can shuffle the pictures, pick from several styles, tweak the layout and contribute to friends collages. But they don’t have to do much work to create a finished product.

Sharing is as important as creation: While Mixel originally helped people create more intricate collages, the iPhone app is much more designed around sharing. Vinh said that Instragram’s example showed that while you first want to make something that looks good, you have to make it easy for people to share. And personal sharing is what drives an app like Mixel.

iPad is a hard place to build a social network: Vinh was hoping to build Mixel into a social network for sharing collages. But he found that while iPad users use the Mixel app for long periods at a time, they don’t come back often enough. The iPhone, with its larger base of users and its ability to go everywhere with users, gives Mixel the ability to be more of a daily app for users. Vinh said 18-20 percent of people are coming back to the app week over week, much higher than the iPad app.

Keep it simple: Vinh said he found it was hard to explain the iPad app without providing a demonstration. That was ultimately part of the problem because it was hard for people to grasp what Mixel was all about.

Next up for Mixel is more tools to make better collages. Vinh doesn’t want to include an image search engine like the iPad app had because that made people focus on outside images. But he is looking at including access to stock images to help people tell fuller stories through their collages.

He’s also trying to use location data so people at the same event can contribute to each other’s albums. Mixel also recently added the ability for people to make their post cards and tag friends in an iPhone 5, iOS 6 optimized version of the app.

Vinh said he was “smarting” a little after the iPad app didn’t take off. But he said he’s learned a lot from the experience and feels he’s positioned to do much better with Mixel’s iPhone app.

“There are now legit reasons to bring friends into the app and to generate value for yourself,” he said. “We didn’t have that before.”

Related research and analysis from GigaOM Pro:
Subscriber content. Sign up for a free trial.

• Survey: How apps can solve photo management
• Social networks will displace business processes, not socialize them
• Sector RoadMap: Social customer service in 2013

“Amazon has now accepted that they will process DMCA notices concerning infringements by Pinterest members,” said a site called Artists’ Bill of Rights, citing a recent email exchange with Amazon.

Amazon is involved because Pinterest, which lets users make pretty collages from photos they find on the internet, uses Amazon’s popular 3S cloud servers to host the Pinterest website:

Amazon has no role in shaping copyright policy at Pinterest but the photographers’ lobby group believes it should nonetheless be responsible because it rents the space that Pinterest is using. According to the site,

Amazon expressed a preference that DMCA notices be submitted to Pinterest on the grounds that it would be more efficient. We are of the view that as the infringing material is actually hosted by Amazon that it is Amazon that should be responsible for dealing with it.

The lobby group may be publicizing Amazon’s role so as to draw media attention to the copyright issue and as an attempt to force the web giant to pressure Pinterest.

Pinterest is still a tiny company but its meteoric growth is forcing it to confront a legal challenge that may be beyond its capacity (Twitter reportedly faced a similar copyright headache when the micro-blog took off in 2009).

Amazon could respond to the copyright controversy by kicking Pinterest off its servers but that seems unlikely given the site’s popularity. Pinterest itself is in a legal grey area reminiscent of that faced by previous pioneers like Grokster and YouTube (NSDQ: GOOG) that helped to create new forms of internet culture.

The hype has led some lawyers to put out alarmist blog posts warning that not just Pinterest but its customers could be liable for copyright infringement.

The Artists’ Bill of Rights claims can be taken with a grain of salt as the site itself appears to be a front for rights ownesr gunning for Pinterest. One of its inaugural posts suggests users approach the site with a caution befitting hard drugs:

Related research and analysis from GigaOM Pro:
Subscriber content. Sign up for a free trial.

• Sector RoadMap: Content personalization in 2013
• Frenemy mine: The pros and cons of social partnerships for online media companies
• Social media in Q1: commerce and discovery dominated