Official source for QGIS icons or banners?

You could go one step up from a QGIS install and pull the icons from the QGIS GitHub repository. PNG (raster) and SVG (vector) icons are available:

https://github.com/qgis/Quantum-GIS/blob/master/images/icons/qgis-icon.png https://github.com/qgis/Quantum-GIS/blob/master/images/icons/qgis_icon.svg

As for GRASS GIS, there are PNG and SVG icons available from their website:

http://grass.osgeo.org/download/logos/

http://grass.osgeo.org/uploads/images/logo/grasslogo_vector_big.png http://grass.osgeo.org/uploads/images/logo/grasslogo_vector.svg

You can find QGIS icons and images included with the installation: C:\Program Files (x86)\Quantum GIS Lisboa\apps\qgis\images\icons

I imagine that this is about as official as you can get.

This page shows you how to reference software and associated files if that is relevant to your presentation.