It has been a really long time since I tried NetBSD for Mac but I don't think that image is a bootable. See
http://www.netbsd.org/ports/mac68k/booter-manual/index.html. The first CD image, by name, should be picked on startup. Try renaming the OS 8 disk to get it higher in priority.
You might also consider the BlueSCSI Discord channel for help.