Show a consistent background image from grub2's boot to unity's login for Ubuntu 16.04.3 with Nvidia GPU

I would like to show a consistent background image from grub2 boot up to unity's login screen for Ubuntu 16.04.3.

Instructions to show a background image I have found so far are:

1. grub2.


2. lightdm and unity-greeter

After implementing them, I noticed that there is still a black screen shown between the grub2 and lightdm stages for a few seconds and a short flash (less than a second) of black screen when transiting between lightdm and unity-greeter.

I think there is a X11 or Xorg stage between grub2 and lightdm which needs setting up. I am also not sure how to avoid the black screen flash between lightdm and unity-greeter.

I have stored an image in /usr/share/backgrounds with root permission and I want to show this image.

How do show this image in X11 or Xorg background? How do I overcome the blackscreen flashing between lightdm and unity-greeter? Is there any stage(s) that I have not identified, and if so, what should I do in those stages to show the image?

Edit: @muru cited another question which asked specifically about customizing the Ubuntu Logo. Plymouth was recommended as a solution to that question. My question is unique to that because it involves a wider scope. Plymouth deals with the splash screen stage, but does not deal with showing an image past the display manager. I am getting a black screen during the splash screen stage and also getting 4 seconds of black between lightdm and unity-greeter (earlier I wrote "a short flash (less than a second)" and this was wrong). The latter issue is beyond the scope of plymouth. Also, I have tried using various plymouth themes and they did not work; I still get a black screen during the splash screen stage.

Edit2: I discovered that different GPU drivers give different results. Below are the drivers that I have tried with a Nvidia GPU connected with a monitor. I also report here the monitor appearances and time (in seconds) after Ubuntu was selected at the Grub2 stage.

dpkg -l | grep nouveau
ii  libdrm-nouveau2:amd64                 2.4.76-1~ubuntu16.04.1   amd64     Userspace interface to nouveau-specific kernel DRM services -- runtime
ii  xserver-xorg-video-nouveau-hwe-16.04  1:1.0.14-0ubuntu1~16.04.1 amd64    X.Org X server -- Nouveau display driver
$ dpkg -l | grep nvidia-384
ii  nvidia-384     384.90-0ubuntu0.16.04.1    amd64    NVIDIA binary driver - version 384.90

Nouveau

Grub2 | Blackscreen |  Plymouth w/ image  | Purple LoginScreen  | LoginScreen w/ image
     2s            14s                   30s                   36s

Nvidia-384

Grub2 |    Blackscreen   | lightdm w/ image |    Blackscreen      | LoginScreen w/ image
     2s                 20s                31s                   37s

In addition, regardless of driver types, a blackscreen would consistently after selecting Ubuntu at the grub2 stage for 12s to 18s. Also my background image would not appear between 30s to 36s time range.

Question: How can I overcome these black and/or purple screens?