Hello,
I have found that I had to place the boot image in the repos directory in order to install debian successfully with razor. I installed with 8 but it should be the same for 7. I found
this doc to be useful and personally ended up downloading the boot image from apt, and copying it into the repos directory. Their docs suggest this should also work with the netboot image. I initially tried supplying razor with one of the ISOs from the download page and encountered the same "cdrom not found" dialog box, and could not work around it by customizing my preseed file. I did end up creating a new task in order to customize my preseed file, working from the built-in task
here. It should also be possible to point to an existing pxe installer if you have one setup already in your environment.
I did run into some issues after this, specifically with the debian installer requiring you select your network interface if there is more than one. I found a bug report about this from 2011, and no fix as of today, which makes unattended installs with razor impossible if you have more than one NIC as a result of the installer bug.