"When you plug an SD card into your Pixel, the app will back up your new photos to Google+ automatically. (For your eyes only, of course.) And when you're offline, you can still view your most recent photos. The Pixel's high resolution display makes your photos look their best, and browsing via touchscreen is a lot of fun. You can share individual photos, set of photos, or an entire album in just a few seconds. Simply select the images you want, then click 'Share.'"
It's a pretty useful app for Chromebooks and it's not limited to the Pixel. In fact, I don't see why it couldn't work on any desktop computer that runs Chrome. It could even replace Picasa at some point.
You can download the app from the Chrome Web Store. Google engineers "are working to bring the app to other Chromebooks as well", even though the app works on other Chromebooks. The Office Viewer powered by the Google-acquired Quickoffice used to be limited to the Chromebook Pixel and it's now available as a Chrome extension.
Update: I've tried to install the app in Chrome for Windows and it worked:
... but then I got this message:
Then I switched to my Samsung Chromebook Series 5, installed the app and it worked well.
Even the SD card import worked:
So the app should work on any Chromebook. Maybe it's only optimized for the Pixel, maybe there are some bugs that need to be fixed. All I know is that the app worked well on my Chromebook and I don't have a Chromebook Pixel.