Custom QR Codes

I have been noticing more and more large brand retailers launch customized QR Code campaigns with an embedded graphic.  It's definitely slick from a brand marketing and mobile marketing standpoint.  So how do they do it?  I did some research and here is what I found (Link, excellent article with lots of samples).

In short, the way it works is that QR codes have up to 30% of error correction built in.  Essentially, a graphic artist would have to overlay the branding element and try to match as many pixel as possible.  It sounds like intense labor but given the high % in error correction, it seems like it isn't all that difficult.  I used GIMP to play around with a QR Code and it worked just fine!