Google+ wasn't the first social network to allow users to embed images directly into comments, but it's supported now and most of us seem pretty happy about it. The only problem with image comments was that a few features were missing from that first implementation. A couple of weeks have passed and we're now looking at a brand new update that fills in most of the gaps: enabling users to open images in full screen, save them locally, and more.
Image comments now open through the viewer Google+ already uses for any other posted images in the app – just tap on the comment and watch it expand. This carries the added benefit that all of the built-in features also apply, including the ability to download or get details about the image, Cast it to a TV, zoom in, and more.
This appears to be the only significant functional change in this version, but keep a look out for other things that may turn up.
The APK is signed by Google and upgrades your existing app. The cryptographic signature guarantees that the file is safe to install and was not tampered with in any way. Rather than wait for Google to push this download to your devices, which can take days, download and install it just like any other APK.