Here you (I :P) go:
+ (UIImage *)imageWithBorderForImage:(UIImage *)image
{
CGFloat radius = 5;
CGSize size = image.size;
radius = MIN(radius, .5 * MIN(size.width, size.height));
CGRect interiorRect = CGRectInset(CGRectMake(0, 0, size.width, size.height), radius, radius);
UIGraphicsBeginImageContext(size);
CGContextRef context = UIGraphicsGetCurrentContext();
CGContextSaveGState(context);
CGMutablePathRef borderPath = CGPathCreateMutable();
CGPathAddArc(borderPath, NULL, CGRectGetMinX(interiorRect), CGRectGetMinY(interiorRect), radius, 1.0*M_PI, 1.5*M_PI, NO);
CGPathAddArc(borderPath, NULL, CGRectGetMaxX(interiorRect), CGRectGetMinY(interiorRect), radius, 1.5*M_PI, 0.0*M_PI, NO);
CGPathAddArc(borderPath, NULL, CGRectGetMaxX(interiorRect), CGRectGetMaxY(interiorRect), radius, 0.0*M_PI, 0.5*M_PI, NO);
CGPathAddArc(borderPath, NULL, CGRectGetMinX(interiorRect), CGRectGetMaxY(interiorRect), radius, 0.5*M_PI, 1.0*M_PI, NO);
CGContextBeginPath(context);
CGContextAddPath(context, borderPath);
CGContextClosePath(context);
CGContextClip(context);
[image drawAtPoint:CGPointMake(0,0)];
CGContextBeginPath(context);
CGContextAddPath(context, borderPath);
CGContextClosePath(context);
CGContextSetRGBStrokeColor(context, 0.5, 0.5, 0.5, 1.0);
CGContextSetLineWidth(context, 1.0);
CGContextStrokePath(context);
CGPathRelease(borderPath);
UIImage *borderedImage = UIGraphicsGetImageFromCurrentImageContext();
CGContextRestoreGState(context);
UIGraphicsEndImageContext();
return borderedImage;
}
Based largely on this question.
One problem is that the border is actually 2px wide (although 1px falls outside of the clip area) because of anti-aliasing. Ideally the border would have ~0.5 alpha, but since the antialiasing gives each of the 2 pixels some alpha, I set it to 1 and it comes out just about right. If I disable antialiasing, then the corners aren't rounded all the same :/