Having had some experience with this, as some clients demand their logo be included on the drawings. As previous repliers have said, the image files are horrendously low quality. The logo appears different, and or in a different location, depending on the machine it was opened on and or printed from, and / or the printer used. I have found the most effective answer is to trace or create the logo in AutoCAD. Most clients cannot discern the difference in the plotted drawing or do not believe the difference is significant enough to concern them. Sorry, i missed your original question: the best format is .wmf (windows metafile), but unfortunately most applications no longer support conversion to this format.
Edited by: ScottHodges on Mar 3, 2010 4:07 PM