In Android there are a collection of Environment paths available at Android.OS.Environment. However, when keeping cross platform capabilities in mind, I want to store pictures
Turns out this isn't available in iOS or Windows phone.
http://docs.xamarin.com/guides/cross-platform/application_fundamentals/building_cross_platform_applications/part_4_-_platform_divergence_abstraction_divergent_implementation (see section 3.1.4).