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<?php
//Default Category Image - if blank set icon
function wpb_default_cat_image( $output ) {

if ( is_category( 'videos' )  &&  ! has_post_thumbnail()  ) : //Set conditionals

	$output = '<a class="video-icon" href="' . get_permalink() .'"><span class="dashicons dashicons-video-alt3"></span></a>'; //change fallback to suit

	return $output;

else:

	return $output;

endif;

}
add_filter( 'genesis_get_image', 'wpb_default_cat_image' );