Here are ways to add photos to the DVD-ROM portion of a disc:
To set iDVD to always add photos automatically:
Choose iDVD > Preferences.
Click Slideshow.
Select the “Always add original photos to DVD-ROM contents” checkbox.
To add slideshow images to the DVD-ROM contents on a case-by-case basis:
Open the slideshow editor by double-clicking the button for your slideshow on the DVD menu, and then click the Settings button (shown below).
Select the “Add image files to DVD-ROM” checkbox.