If you want to round four corners of an image in your iPhone Application, the following piece of code should help.

void addRoundedRectToPath(CGContextRef context, CGRect rect, float ovalWidth, float ovalHeight)
{
	float fw, fh;
	if (ovalWidth == 0 || ovalHeight == 0) {
		CGContextAddRect(context, rect);
		return;
	}
	CGContextSaveGState(context);
	CGContextTranslateCTM (context, CGRectGetMinX(rect), CGRectGetMinY(rect));
	CGContextScaleCTM (context, ovalWidth, ovalHeight);
	fw = CGRectGetWidth (rect) / ovalWidth;
	fh = CGRectGetHeight (rect) / ovalHeight;
	CGContextMoveToPoint(context, fw, fh/2);
	CGContextAddArcToPoint(context, fw, fh, fw/2, fh, 1);
	CGContextAddArcToPoint(context, 0, fh, 0, fh/2, 1);
	CGContextAddArcToPoint(context, 0, 0, fw/2, 0, 1);
	CGContextAddArcToPoint(context, fw, 0, fw, fh/2, 1);
	CGContextClosePath(context);
	CGContextRestoreGState(context);
}

- (UIImage *)roundCornersOfImage:(UIImage *)source {
	int w = source.size.width;
    int h = source.size.height;

	CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
	CGContextRef context = CGBitmapContextCreate(NULL, w, h, 8, 4 * w, colorSpace, kCGImageAlphaPremultipliedFirst);

	CGContextBeginPath(context);
	CGRect rect = CGRectMake(0, 0, w, h);
	// Set the oval width and height to be quarter of the image width and height
	addRoundedRectToPath(context, rect, w/4, h/4);
	CGContextClosePath(context);
	CGContextClip(context);

	CGContextDrawImage(context, CGRectMake(0, 0, w, h), source.CGImage);

	CGImageRef imageMasked = CGBitmapContextCreateImage(context);
	UIImage *newImage = [[UIImage imageWithCGImage:imageMasked] retain];

	CGContextRelease(context);
	CGColorSpaceRelease(colorSpace);
	CGImageRelease(imageMasked);

	return newImage;    
}

Example: Try to show a rounded image in a UIView

// Create an UIImageView with size 96 x 96
UIImageView *aImageView = [[UIImageView alloc] initWithFrame:CGRectMake(0, 0, 96, 96)];

// Round the image and set it in the UIImageView
aImageView.image [self roundCornersOfImage:<UIImage>];

Done =)

Reference: Rounded corners to images