MKPinAnnotationViewでなく、MKAnnotationViewを使います。
Before
-(MKAnnotationView*)mapView:(MKMapView*)mapView
          viewForAnnotation:(id)annotation{
    
    CustomAnnotation* ca = (CustomAnnotation*)annotation;
    
    static NSString *PinIdentifier = @"Pin";
    MKPinAnnotationView* av = (MKPinAnnotationView *)[mapView 
                    dequeueReusableAnnotationViewWithIdentifier:PinIdentifier];
    
    if (!av) {
        av = [[MKPinAnnotationView alloc] initWithAnnotation:annotation
                    reuseIdentifier:PinIdentifier];
        av.canShowCallout = YES;
        av.rightCalloutAccessoryView = [UIButton buttonWithType:UIButtonTypeDetailDisclosure];
    }
    
    av.image = [UIImage imageNamed:@"image.png"];
    av.annotation = annotation;
    
    return av;
}
After
-(MKAnnotationView*)mapView:(MKMapView*)mapView
          viewForAnnotation:(id)annotation{
    
    CustomAnnotation* ca = (CustomAnnotation*)annotation;
    static NSString *PinIdentifier = @"Pin";
    MKAnnotationView * av = (MKAnnotationView  *)[mapView
                    dequeueReusableAnnotationViewWithIdentifier:PinIdentifier];
    
    if (!av) {
        av = [[MKAnnotationView alloc] initWithAnnotation:annotation
                    reuseIdentifier:PinIdentifier];
        av.canShowCallout = YES;
        av.rightCalloutAccessoryView = [UIButton buttonWithType:UIButtonTypeDetailDisclosure];
    }
    
    av.image = [UIImage imageNamed:@"image.png"];
    av.annotation = annotation;
    
    return av;
}