There is no way to take your Timeline content to an ISO Image in Premiere Elements 12/12.1 on Windows 8.1 64 bit with just Premiere Elements 12/12.1 by itself.
Premiere Elements 13 is the first to offer a Burn To: option of ISO Image for DVD, AVCHD, and Blu-ray. Which one of those choices are you seeking (or all of them)?
If you cannot make due with the Burn To: Folder for DVD and AVCHD, then you can get your ISO Image from 12/12.1 in the following round about way....
Let us do AVCHD ISO Image as the example...
1. Burn your Premiere Elements 12/12.1 Timeline using Publish+Share/Disc/AVCHD, Burn To: = Folder (4.7 GB).
2. Take the BDMV Folder from the saved folder into the free ImgBurn program and export is as a ISO file.using the category named
Create Image File from Files/Folder.
3. When you need the disc product, you can take that ISO file to disc in ImgBurn using the category named Write Image File To Disc.
For more information on ISO Image and Premiere Elements, please review the following link's content
After you review and consider the above, we can discuss the matter further if needed.
Downloading and installing the free ImgBurn needs caution since it is known to travel with unwanted carry alongs.