"make new image..." in iDVD problem

I’m having real trouble with a script that I’m trying to write for importing large amounts of photos from iPhoto to iDVD. I’m nearly finished, but I can’t figure out why this section of script doesn’t work.

I have consulted Apple’s example iDVD scripts, to no avail. When I run the following script I get an error: “iDVD got an error: NSContainerSpecifierError” and Script Editor highlights the “make new image…” line.

Can anyone help me to figure out what I’ve got wrong?