Try this markdown:

![alt text](http://url/to/img.png) 

I think you can link directly to the raw version of an image if it's stored in your repository. i.e.

![alt text](https://github.com/[username]/[reponame]/blob/[branch]/image.jpg?raw=true) 

You can also use relative paths like

![Alt text](relative/path/to/img.jpg?raw=true "Title") 

Also try the following with the desired .fileExtention:

![plot](./directory_1/directory_2/.../directory_n/plot.png) 

• You can create a New Issue
• upload(drag & drop) images to it
• Copy the images URL and paste it into your README.md file.

here is a detailed youTube video explained this in detail:

https://www.youtube.com/watch?v=nvPOUdz5PL4

It's much simpler than that.

Just upload your image to the repository root, and link to the filename without any path, like so:

![Screenshot](screenshot.png) 

You can also add images with simple HTML tags:

<p align="center">   <img src="your_relative_path_here" width="350" title="hover text">   <img src="your_relative_path_here_number_2_large_name" width="350" alt="accessibility text"> </p>