Condition for category Image

you can create similar conditional placeholder using this tutorial – https://wp-events-plugin.com/tutorials/creating-conditional-placeholders-for-events/ (changing event to category and then $EM_Event to $EM_Category)

e.g.

if( in_array($matches[1],$EM_Event->styles) ){

to

if( $EM_Category->get_image_url() == '' ) {

*paste in your theme functions.php

OK. looks like this will do it. Thanks.