When we were visiting Yahoo several weeks ago, the walls were plastered with posters about a compelling-looking product called PhotoMail. No one wanted to talk about it at the time, but today the company finally went public with it.

What is it? It’s an interesting way to share photos via email. Instead of emailing the actual photos, PhotoMail inserts small thumbnails of the photos in the body of the email messages, and uploads the full-size images to the Yahoo Photos album service. Recipients get formatted email messages with thumbnails that link to the full-size images. When they click on an image they want to view, they’re taken to a temporary online album that Yahoo has automatically created.

The new feature – it’s being integrated into Yahoo’s email service – is smart on many levels.

Because the full-size images are stored on the Web, it makes it easy to send dozens or even hundreds of images via email without worrying about clogged in-boxes. Yahoo, which spends countless hours studying user behavior and feedback, has made the service very easy-to-use, based on the demo we saw. The feature also makes smart business sense, because it will drive more users to Yahoo Photos, potentially keeping people on the Yahoo network that much longer. Recipients of the email messages will also be encouraged to become Yahoo members and create Yahoo Photos accounts.

We do have a big gripe, though. The service requires users to download a small application to upload the photos, and Yahoo is relying on the ActiveX functionality of Internet Explorer for its upload software. That means the feature is limited to Windows IE users. Many of us stopped using IE in favor of Firefox long ago, and I doubt many will turn back to IE just to email photos to friends and family. Andy Spillane, VP of Yahoo Mail, says the company is exploring using XUL to build a Firefox version of the software.

As nice as PhotoMail is, what we really want to know is when Yahoo will roll out a new user interface for Yahoo Mail. Ever since Yahoo acquired San Francisco start-up Oddpost last year, speculation has been rampant that the company will unveil a mail interface that rivals Google’s Gmail for usability. Spillane wouldn’t divulge details about that. But he said to expect two or three more big announcements by Labor Day. Clearly, something is in the works, and Spillane can barely contain his excitement about it.

“This is a big deal,” he said of Yahoo PhotoMail. “But we have two or three other things that are as big or bigger….There will be no question who’s the leader in email innovation.”