PicPlz, a photo-sharing start-up, has released a programming interface that lets applications tap into its tools for uploading and applying artistic filters to images.

"We think that allowing developers access to our upload and filter pipeline brings something different to the table than "just another photo-sharing API,'" the company said in a blog post yesterday. "We're pleased to announce that in the past 2 weeks we've had well over 100 developers apply to be part of our API (far exceeding our expectations)."

Opening an API lets programmers tap into the abilities of a Web site or operating system, turning it into a foundation for a broader ecosystem. PicPlz released sample code, for example, for building a slideshow that displays PicPlz images.

APIs are important for growth, too. By exposing an interface, as Twitter did years ago, a successful service can become plumbing for others' services. That can complicate Web business plans, though: it's hard to show ads if nobody is coming to your Web site. But if your service becomes a utility others depend on, you can charge for high-reliability, premium access to its interface.

For years, photo sharing has been dominated by Yahoo's Flickr and some other relatively slow-moving sites such as Shutterfly and Google's Picasa Web Albums. But the old order is being overturned with people sharing photos with Facebook, Instagram, and other tools that make the process relatively easy and very social.