acf_validation_error_messages

add_filter('acf/validate_value/name=validate_this_image', 'my_acf_validate_value', 10, 4);

function my_acf_validate_value( $valid, $value, $field, $input ){
	
	// bail early if value is already invalid
	if( !$valid ) {
		
		return $valid;
		
	}
	
	
	// load image data
	$data = wp_get_attachment_image_src( $value, 'full' );
	$width = $data[1];
	$height = $data[2];
	
	if( $width < 960 ) {
		
		$valid = 'Image must be at least 960px wide';
		
	}
	
	
	// return
	return $valid;
	
	
}