The Above video says it all.
Whenever you come across an "Editor Box" inside Easywebinar and adding Images requires a "source", it is actually looking for a URL that ends with .jpg / .png
Usually you can get that URL by uploading your Image to a hosting site such as AmazonS3, Wordpress Media Folder, Pixabay, Giphy etc.
The below steps will be your alternative if you want to upload your Photo into Easywebinar's Media Library instead..