The format of DVDs is such that they will alway be less sharp than on your monitor because the image is compressed substantially. That said, users have some control over the amount of image compression and the "engine" which does the compression. In iDVD, set your Preferences to "Best Performance" or "Pro" for better looking images (better compression management).
You can also try the Ken Burns effect to pan and zoom, and give your viewer a different experience than a straight slideshow.
Also, you could consider dedicated slideshow software like Photo To Movie or Fotomagico.
Search in these forums for lots of thoughts on how best to retain photo quality.
John