Go to the Advanced menu in iDVD and select the Edit DVD-ROM Contents... option.
In the window that comes up you can click on New Folder button and name it Slideshow Photos.
Then click on the Add Files... button to add your selected phiotos to that folder. It will be included on the DVD disk when the project is burned as long as there is enough space on the DVD disk.
I suggest you following this workflow to help assure the highest quality burned disk possible:
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.
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