Q: Why the main image is not loading in my template?
A: Be sure you have not done some manipulation to your template and somehow replaced the image thumbnail that was present with the merge tag code for an image on the front end of the editor.
The merge tag is {{main_image}} and if you notice that code (above the template box), is inside of HTML code. That code has to be put in on the back end (in your source code) and not on the front end... otherwise, you're telling the software to just "display" the image URL on the front end instead of displaying the actual image.
You'll need to reset your template by selecting a different template. The code for the image is already there and doesn't need to be modified.