What is the best format to use when inserting logos into a map document while maintaining transparency?

Answer

The best format for inserting logos into map documents is .EMF.

Logos are usually designed in a vector illustration package such as Adobe Illustrator, and then exported to a pixel-based format. However, exporting the logo as EMF will maintain the vectors and allow resizing in ArcMap without loss of quality. It will display correctly in ArcMap when printed and exported since there is no background to a vector drawing.