Great tut !!
Also, be careful when editing the file, the header must remain unedited because this can corrupt the file into not opening anymore ..
You recognize it by it's content, such as
Adobe Photoshop CS5.1 Windows, xmp:ModifyDate>2012-11-29T17:55:57+01:00</xmp:ModifyDate, ...
and so on. It's in the beginning of the image's code.
Happy glitching !