How do I install Ubuntu on a VirtualBox client from an ISO image?

I would venture to say you are overcomplicating things! You only need to mount the iso in the virtual machine.

  1. Right click on the VM you want to run the .iso in
  2. Click on 'Storage' (On Oracle VM virtualBox, Click on 'Settings' first to get to 'Storage')
  3. Under IDE Controller There should be an icon that shows a CD with a + sign on it, to create a new disc drive as shown highlighted in yellow color in the below snapshot:

    Ubuntu - Settings, Storage menu

  4. A box will come up, click Choose Disk

  5. Choose your .iso

Should boot into the iso like it's in the disc drive.