There are actually many ways to achieve the goal.
- Install ES File Explorer -- I'm suggesting it because I know that it provides a file widget.
- In your default launcher, go to Widgets → choose ES File Explorer → locate and select the image you want to be launched by the shortcut, and it would be placed on your home screen now.
You already know it.
Open your browser, enter
file:/// in the URL bar → locate to the folder where the image is located and then tap on it so that the browser should load it (show it, not download it) → use "Add shortcut to home" so that the shortcut can be created on the home screen.
Note that I tested this method on Firefox for Android and works well, but Chrome doesn't work.
Find the activity responsible for showing the image in your default gallery app, input the data (image URI) into it, and use them in the Tasker to create a shortcut. (Use my answer for know-how.)