Users don't need to have the Xim app to see shared photos. But Xim does require (for now) a U.S. phone number, allowing users to use email to "Xim" to anyone with a smartphone. The Xims are viewable in browsers and expire after about an hour.
To create a Xim, a user does need the Xim app. Starting today, October 8, Xim apps are available for Windows Phones and Android phones; an iOS version is coming soon. The Xim creator can choose anywhere from one to up to 50 photos from a camera roll or another third party service like Dropbox, Facebook or Instagram, and then choose whom to invite via a text or emailed invitation.
Microsoft did make WindUp — which allowed sharing of images, videos, text and audio snippets for "a finite period of time" — available to Windows Phone users. It's not clear from Microsoft's blog post if Xim built on any of WindUp's concepts or technologies.