So you started on an ISO of 8 and did the upgrade and now you want to make an ISO or you cannot make it yourself. If you explain exactly what you started from I may be able to send a link. From what you are describing I do not thing the free upgrade was supposed to work with buying a lic. See below:
If you installed Windows 8.1 Preview by booting from ISO media, you can update to the final version of Windows 8.1 Pro using the Windows Store, but your system won't activate until you buy a Windows 8.1 license and product key. You can do this by following the instructions when you activate your system, or by purchasing a DVD of Windows 8.1 Pro.
Because I like Windows 8 after I used for a while, I actually bought new laptop from Officemax. My laptop comes with recovery hidden partition and I did a backup to my usb flash drive before I upgraded to 8.1 online. I also did a shrink volume to create a D: partition. Now, I see a lot hidden partitions and I don't know if my recovery media can restore back to Windows 8 factory default in my case. I am worry I don't have the ISO for backup after I upgraded to Windows 8.1.