This Plugin will take a screenshot, and save / register the image file so that it appears in your picture gallery (camera roll) on iOS and Android. Does all the dirty work so you don't have to waste valuable coding time!
The plugin can also register an existing image (i.e. a Texture2D loaded from your Unity resources folder) to the gallery.
Tested and working with iOS 5/6/7 + Android 2.3.x + 4.x