gistfile1.m

- (UIImage *)addBorderToImage:(UIImage *)image {
	CGImageRef bgimage = [image CGImage];
	float width = CGImageGetWidth(bgimage);
	float height = CGImageGetHeight(bgimage);
	
        // Create a temporary texture data buffer
	void *data = malloc(width * height * 4);
	
	// Draw image to buffer
	CGContextRef ctx = CGBitmapContextCreate(data,
                                                 width,
                                                 height,
                                                 8,
                                                 width * 4,
                                                 CGImageGetColorSpace(image.CGImage),
                                                 kCGImageAlphaPremultipliedLast);
	CGContextDrawImage(ctx, CGRectMake(0, 0, (CGFloat)width, (CGFloat)height), bgimage);
	
	//Set the stroke (pen) color
	CGContextSetStrokeColorWithColor(ctx, [UIColor greenColor].CGColor);

	//Set the width of the pen mark
	CGFloat borderWidth = (float)width*0.05;
	CGContextSetLineWidth(ctx, borderWidth);

	//Start at 0,0 and draw a square
	CGContextMoveToPoint(ctx, 0.0, 0.0);	
	CGContextAddLineToPoint(ctx, 0.0, height);
	CGContextAddLineToPoint(ctx, width, height);
	CGContextAddLineToPoint(ctx, width, 0.0);
	CGContextAddLineToPoint(ctx, 0.0, 0.0);
	
	//Draw it
	CGContextStrokePath(ctx);

        // write it to a new image
	CGImageRef cgimage = CGBitmapContextCreateImage(ctx);
	UIImage *newImage = [UIImage imageWithCGImage:cgimage];
	CFRelease(cgimage);
	CGContextRelease(ctx);
	
        // auto-released
	return newImage;
}

Works great, but dont forget to add this:

free(data);