There are actually many ways to achieve the goal.
###Method 1
- 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.
###Method 2
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.
###Method 3
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.)