You created the slideshow in iDVD by importing still imaged into iDVD, righrt? What size were the images you used in the iDVD slideshow? Were they sized to a 4:3 ratio as used by iDVD? What transition did you apply?
DVD video by design reduces all media to 640 x 480 pixels, standard TV resolution. They will never look as sharp as when viewed on a computer screen.
xvideo DVD:
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, launch DVD Player and play it. If it plays OK with DVD Player the encoding is good.
Then burn to disk with Disk Utility or Toast at the slowest speed available (2x-4x) to assure the best burn quality. Always use top quality media: Verbatim, Maxell or Taiyo Yuden DVD-R are the most recommended in these forums.
OT