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..
In your Event Editor, look for any Media Library box to upload your Photo
Add or upload the image to the library but not actually using it. (Just upload it)
Once uploaded, right click on that image and click
"Copy Image Address/Location"
Paste that address in the Source Field :