I want to insert images in a slider in my README.md
I have inserted a single image like this

There is no way to display a slideshow in Github Flavored Markdown.
If you want to display multiple images without making your README excessively long, you can use a <details>
tag:
<details>
<summary>The title of the image group</summary>
<img src="image-url" alt="image-description"/>
<img src="image-url" alt="image-description"/>
<img src="image-url" alt="image-description"/>
</details>
The images will be grouped in a widget that can be clicked to show or hide the content inside: