So far Google released two versions of Android 5.1 Factory Image and today third version LMY47I replaces the ongoing LMY47E version. The LMY47E version of Android 5.1 was the replacement of first LMY47D.
If you don’t know how you can use factory Image, let me clear it for you. The factory Image of OS used by those Nexus users who wants to restore the stock version of the Android OS on their devices. It’s also useful when user misses some important update, they can update using factory image.
In case, if Nexus user lost their current OS while doing an attempt to customize, they can bring their device back to stock Android by using factory image of Android version. Well if you don’t want to wait for official OTA update, you can update device straight away by flashing latest Android version factory image.
Currently factory image of latest Android 5.1 is available for both Nexus 5 and Nexus 6. If you still waiting for Android 5.1, you can flash factory image on your nexus 5 and nexus 6.