This is available out of the box. You just need to create a custom filed with the "jcalpromedia" field type. After that create new Event form and assign the field from previous step to that form. When you have done that, you need to assign that form the the specific event category. Your event creation form will now have new tab which will include defined field and you will be able to choose an image for the event using it

You can't extend the default event form. You have to assign a new form to the category

WE LOVE JOOMLA

Joomwalker is not affiliated with or endorsed by the Joomla Project or Open Source Matters.
The Joomla name and logo are used under a limited license granted by Open Source Matters the trademark holder in the United States and other countries.