[Answered ]-Django how to detect if a form field was filled in?

You may want to look at a custom ImageField, which would handle your custom processing in its clean method. Take a look at this for instance: https://djangosnippets.org/snippets/2206/

I would do it in the form class itself by overriding the clean method and checking it.
when you call is_valid() from your view then it will inturn call the clean method where you can do this checking.