Hi,
two render engines exist together:
1. Presenter: the old engine. only COM API provides something. You can refer to the SDK sample to get an idea.
C:\Program Files\Autodesk\Navisworks Manage 2014\api\COM\examples\Auto\AutoPresenterExample
2. OGS: the tab [Render] in UI. It is the new engine. NO any API
As to export picture, you can refer to this sample:
C:\Program Files\Autodesk\Navisworks Manage 2014\api\COM\examples\Auto\AutoDrivePluginExample
The two samples above are Automation of COM API. You can use them by COM interop in .NET.