Hello, I just wanted to know about video editing since I am new to this and would like to know how to add an image as a background to a video after it has been made. I do not know what software I will need and if possible please mention any free software that would be useful. Thank you.

Your post is too vague. Please say exactly what you need to do. For example: Are you talking about background image at the menu stage before the video starts or what?

Sorry, what I want to take a short video that I have and add like a podium and a background to the person talking. The podium and the background would just be any image I need. I hope that helps.