When you've got a CLLocationCoordinate2D coordinate and want to plot a dot on a image of a map that you know its NW and SE coordinates. In my case the image is 100x100px.

plot-a-dot-on-map.m

    CLLocationCoordinate2D sightingLocation = CLLocationCoordinate2DMake([self.sighting.locationLat floatValue], [self.sighting.locationLng floatValue]);
    
    // Australia's map coordinates
    CLLocationCoordinate2D nw, se;
    nw = CLLocationCoordinate2DMake(-7.410849283839832, 112.41210912499992);
    se = CLLocationCoordinate2DMake(-44.248667520681586, 154.07226537499992);
    
    MKMapPoint topLeftPoint = MKMapPointForCoordinate(nw);
    MKMapPoint bottomRightPoint = MKMapPointForCoordinate(se);
    MKMapRect mapRect = MKMapRectMake(topLeftPoint.x, topLeftPoint.y, bottomRightPoint.x - topLeftPoint.x, bottomRightPoint.y - topLeftPoint.y);
    MKMapPoint targetPoint = MKMapPointForCoordinate(sightingLocation);
    
    BOOL isInside = MKMapRectContainsPoint(mapRect, targetPoint);
    
    if (isInside)
    {
        double left = (targetPoint.x - topLeftPoint.x) / (bottomRightPoint.x - topLeftPoint.x);
        double top = (targetPoint.y - topLeftPoint.y) / (bottomRightPoint.y - topLeftPoint.y);
        
        CGFloat diameter = 10;
        CGFloat radius = diameter / 2;
        CGRect frame = CGRectMake(left * 100 - radius, top * 100 - radius, diameter, diameter);
        UIView *circle = [[UIView alloc] initWithFrame:frame];
        circle.alpha = 0.5;
        circle.layer.cornerRadius = radius;
        circle.backgroundColor = [UIColor IORedColor];
        
        [self.mapImageView addSubview:circle];
    }