Also instead of burning to disk follow this workflow to help assure the best qualty video 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.
Here's some food for thought: iDVD has been discontinued and unsupported for about 6 or so years. It is becoming more and more incompatible with the current OS X systems. You might consider migrating to a supported video DVD authoring application.
The App Store has a number of potential apps listed. Most just handle movie/video files but one, DVD Creator Lite, can also create slideshows from still photos as well as use movie/video files .