There are actually many ways  to achieve the goal.

###Method 1

1. Install [ES File Explorer](https://play.google.com/store/apps/details?id=com.estrongs.android.pop) -- I'm suggesting it because  I know that it provides a file widget.
2. 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](https://play.google.com/store/apps/details?id=net.dinglisch.android.taskerm) to create a shortcut. (Use my [answer](https://android.stackexchange.com/a/113923/96277) for *know-how*.)