The Standard Definition (SD) and High Definition (HD) photo selector allows users to toggle between SD and HD photos where HD photos are available. This feature is provided to help users control data usage. HD photo viewing consumes additional data.
If the HD box in the lower right corner has a "\" through it then this means that HD is not an option for this particular photo.
This selector is available on Android tablet devices when viewing full-size images and when the property contains HD images. It is available on Android phones when the device is turned to landscape view (slideshow) and the property contains HD images.
The HD icon box is highlighted blue when viewing the photo in HD. When it is turned off, the HD icon box is clear.