I'm pretty sure you created the image wrong. When you create the image in Disk Utility, it's very important that you specify the source correctly. I'm not certain I remember exactly right, so you should test this before you trust the results, but here's the process as I remember it:
In Disk Utility's sidebar, select the DVD's entry. IIRC, it'll actually list two entries in the sidebar: the DVD itself, and then indented under that the installer volume ("Mac OS X Install DVD" or something like that. You want the DVD entry, not the volume entry under it.
IIRC older versions of Disk Utility would actually list three entries: the DVD, a session under that, and the volume under that. In this case, I'm pretty sure it's the session (the middle entry) you want.
Under the File menu, choose "New" > "Image from disk2" (or whatever the disk ID is). This tells Disk Utility you want a raw device image, not just image the files contained in the disk.
In the save dialog, set the Image Format to "DVD/CD master"
I think that procedure will work, but to be sure it's best to rename the file from ".cdr" to ".iso", copy it to your Windows computer, burn it to a DVD-R, and actually try booting the Mac from it. Note that if you mount the .iso on Windows you will not see anything like what you expect -- IIRC the 10.5 install DVDs are actually in a hybrid format that contains a Windows-readable driver disk for Boot Camp, and a Mac-readable (& bootable) installer volume. So if you mount it on Windows, you'll see Windows drivers, not a Mac installer. (BTW, this is also true of the original DVD, so try mounting that on your Windows computer to check my memory.)
Best Answer
There's a script for that:
It's a pretty straight forward process, finding the bootable portion of the installer, making temporary files, calling
asrto restore to the temporary image, convert to ISO, and then name the files with the version of the OS. With all the changes from 10.7 to 10.12 - there are some special cases in the script, but it's quite general in that it works and you can see what it's doing should it break in the future and Rich and the team that collaborate don't find and fix things before we do.