How to use custom icons with mapKit framework?

You can change the annotation image by using following code,

- (MKAnnotationView *) mapView:(MKMapView *)mapView viewForAnnotation:(id <MKAnnotation>) annotation
{
    if ([[annotation title] isEqualToString:@"Current Location"]) {    
        return nil;
    }

    MKAnnotationView *annView = [[MKAnnotationView alloc ] initWithAnnotation:annotation reuseIdentifier:@"currentloc"];
    if ([[annotation title] isEqualToString:@"McDonald's"])
         annView.image = [ UIImage imageNamed:@"mcdonalds.png" ];
    else if ([[annotation title] isEqualToString:@"Apple store"])
         annView.image = [ UIImage imageNamed:@"applestore.png" ];
    else
         annView.image = [ UIImage imageNamed:@"marker.png" ];
    UIButton *infoButton = [UIButton buttonWithType:UIButtonTypeDetailDisclosure];   
    [infoButton addTarget:self action:@selector(showDetailsView) 
             forControlEvents:UIControlEventTouchUpInside];
    annView.rightCalloutAccessoryView = infoButton;
    annView.canShowCallout = YES;
    return annView;
}

where "marker.png" is your image file.