Put a transparent button over that portion
update
-(void)singleTapImageview:(UITapGestureRecognizer *)gesture
{
NSLog(@"single-tap");
CGPoint touchLocation = [gesture locationInView:self.imageViewParent];
float x = touchLocation.x;
float y = touchLocation.y;
//Using gesture recogniser you can get the current position of touch
UIImage* image = imageViewParent.image;
CFDataRef data = CGDataProviderCopyData(CGImageGetDataProvider(image.CGImage));
UInt8 *pixels = (UInt8 *)CFDataGetBytePtr(data);
UIGraphicsBeginImageContext(image.size);
[image drawInRect:CGRectMake(0, 0, image.size.width, image.size.height)];
CGContextSetLineCap(UIGraphicsGetCurrentContext(), kCGLineCapRound);
CGContextSetLineWidth(UIGraphicsGetCurrentContext(), 1);
CGContextSetStrokeColorWithColor(UIGraphicsGetCurrentContext(), [UIColor redColor].CGColor);
int index = ((x + (y * (int)image.size.width)) * 4) + 3;
CGFloat alpha = pixels[index];
if (alpha == 0)
{
// call button acton
}
}