Mail merge- Sending an HTML email

I have created an HTML Template to blast over email. I have an <img src> tag which I would like for it to be updated as a merge field from my excel data sheet. 

The HTML Page was created by a third party online service, and to load it for mail merge, I simply open it in word. I do not know if I can view the source code in word and insert placeholders in it, and even if I can, I dont know if I can do it for image source in the code. 

Please help me out. My first query in the community. 

Thanks in advance.