If you've already installed the app in Chrome, now you can run it: just click the Google+ Photos icon from the new tab page or the app launcher and then click "run anyway". Until now, the "run anyway" button wasn't available.
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Here's the Google+ Photos app in Windows 8:
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Unfortunately, the app's page from the Chrome Web Store only lets you install the app in Chrome OS, at least officially. For example, if you've installed the app on a Chromebook and you use Chrome Sync, then it should also be installed on your other computers that run Chrome.
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How to install the app in Chrome? First enable "Experimental Extension APIs" in the chrome://flags page and restart the browser. Go to this page, right-click the "Add the Chrome" button, select "Inspect element", remove the "webstore-button-disabled" class from the code (use Ctrl+F and search for webstore-button-disabled, double-click the value of the class attribute, delete "webstore-button-disabled" and press Enter), then click "Add to Chrome".
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{ Thanks, Andrea Leardini, Tim Nguyen and Stef van der Woerdt. }