I have an 8.1 ISO, except fully expanded. I can't run setup because it asks me for a Windows 8.1 key and will not accept my 8 key. I've tried various dism options. Nothing happened, but I was never confident that what I ran was correct. If you'd like to make a suggestion as far as what I should run on the command line I'd be happy to try it. The 8.1 files live at H:\Windows81\ (e.g., that's where the setup.exe file is located).
I tried something similar with an ISO that I have using Powershell's equivalent to DISM and got a "not authorized". FWIW I had noticed that it had an Install.WIM and was just trying to list its contents
OMG. I can get into it! Try this:
| PS E:> Get-WindowsImage -ImagePath "Sources\Install.wim" |
|---|
So, that would be H: in your case. I think where I went wrong after that is not understanding how to use the Index parameter. But the output of this command at least now hints what it means. See what you can get that way?