From the looks of the OOXML format, the image there is just used as a resource. http://web.mit.edu/~stevenj/www/ECMA-376-new-merged.pdf xl/theme/_rels/theme1.xml.rels refers to image1.png. I'd say it's a default background of some description. Still, it would be cool if there was something in there. Maybe taking a look at the frequency of the bytes would help?