Short answer
You can add an image this way:
<img src="https://url_of_the_image"/>
Long answer
Step 1: Update your images to Shopify
You need to first upload your image onto your Shopify admin. It has a section called “files", you can upload your image there.
Then, you will get a URL of the uploaded image. Now your image is on Shopify server.
Step 2: Replace with the new image URL
Then replace “url_of_the_image” with the new image URL.
And this new string of the HTML code can be added to your product description to have this image shown in your product description area.
Want to replace a chunk of HTML descriptions in your product description? Check this guide: How to bulk replace my product description