Technically, your file is not an ISO, but simply an image of an ext4 partition that you have exported as a file with a .iso extension, but remember, it is not actually an iso (iso files are disk images, but not all disk images are iso's).
instead you need to use DD to write the file back out to a volume, and then you will be able to leverage tools like photorec.
I'd recommend using a virtual machine running a linux flavor from live cd, so you can create a virtual hdd file for writing your partition out to, without having to allocate physical storage to the task.
also if you find many of your files unrecoverable from your image, try taking a new image using ddrescue. it takes a very long time, but it can be worth it if your files are valuable enough to you.