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.
This might help: iDVD 7.0 Help (iLife '08): Preparing images for iDVD slideshows. If you intend to use the Ken Burns effect then I'd doulbe the size recommended in the above article.
Once you have the project as you want it save it as a disk image via the File ➙ Save as Disk Image menu option. This will separate the encoding process from the burn process. To check the encoding mount the disk image and launch DVD Player and play it.
If it plays OK with DVD Player the encoding was good and you can then burn to disk with Disk Utility at the slowest speed available, 2x-4x, to assure the highest burn quality. Always use top quality media: Verbatium, Maxell or Taiyo Yuden DVD-R are the most recommended in these forums.