iphone Mask an an irregular Image but keep a border

Question

Hi there,
My app is caching images perfectly but I'd like to add a border to the masked image. I'm currently using the following code :

CGImageRef CopyImageAndAddAlphaChannel(CGImageRef sourceImage) {
    CGImageRef retVal = NULL;

    size_t width = CGImageGetWidth(sourceImage);
    size_t height = CGImageGetHeight(sourceImage);

    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();

    CGContextRef offscreenContext = CGBitmapContextCreate(NULL, width, height, 
                                                          8, 0, colorSpace, kCGImageAlphaPremultipliedFirst);

    if (offscreenContext != NULL) {
        CGContextDrawImage(offscreenContext, CGRectMake(0, 0, width, height), sourceImage);

        retVal = CGBitmapContextCreateImage(offscreenContext);
        CGContextRelease(offscreenContext);
    }

    CGColorSpaceRelease(colorSpace);

    return retVal;
}


// masker method
- (UIImage*)maskImage:(UIImage *)image withMask:(UIImage *)maskImage {
    CGImageRef maskRef = maskImage.CGImage;
    CGImageRef mask = CGImageMaskCreate(CGImageGetWidth(maskRef),
                                        CGImageGetHeight(maskRef),
                                        CGImageGetBitsPerComponent(maskRef),
                                        CGImageGetBitsPerPixel(maskRef),
                                        CGImageGetBytesPerRow(maskRef),
                                        CGImageGetDataProvider(maskRef), NULL, false);

    CGImageRef sourceImage = [image CGImage];
    CGImageRef imageWithAlpha = sourceImage;
    //add alpha channel for images that don't have one (ie GIF, JPEG, etc...)
    //this however has a computational cost
    if (CGImageGetAlphaInfo(sourceImage) == kCGImageAlphaNone) { 
        imageWithAlpha = CopyImageAndAddAlphaChannel(sourceImage);
    }

    CGImageRef masked = CGImageCreateWithMask(imageWithAlpha, mask);

    CGImageRelease(mask);

    //release imageWithAlpha if it was created by CopyImageAndAddAlphaChannel
    if (sourceImage != imageWithAlpha) {
        CGImageRelease(imageWithAlpha);
    }

    UIImage* retImage = [UIImage imageWithCGImage:masked];
    //UIImage* retImage = [UIImage imageWithCGImage:maskedTransparent];
    CGImageRelease(masked);

    return retImage;
}

The masks used are irregular and I'd like to keep a border around the masked image ...

Currently, my masks are black and white only. I tried with an additional red border but it was masked as well : red is transformed to transparent.... any idea about how to do it ?