DVD is as standard - only interlaced SD-Video (as on old CRT-TVs) and can never be any better than this.
This is less than a quarter of Your Mac screen resolution.
That's one part of Why You can see High res. on Your Mac - and not on DVD.
Next part is what program is used to make the SlideShow.
If You use iMovie'08 or 09 or 11 - to make it - it too degrades the final result as non of them can deliver interlaced SD-Video to iDVD but only half (every second line in the picture)
• iMovie HD6 or
• FinalCut (any version) or
As they all delivers 100% of what iDVD or Toast™ or DVD Studio Pro or Whatever needs.
Yours Bengt W