Strange behaviour with watermark

[filippobistaffa]

Steps to reproduce the problem:

1. download this PNG file in the "examples" folder (same width and height of "watermark-polito.png")
2. change "watermark" option in "chameleon.tex" from "watermark-polito" to "iiia"
3. compile normally, no watermark should appear
4. rename "iiia.png" to "torino.png"
5. change "watermark" option in "chameleon.tex" from "iiia" to "torino"
6. re-compile, now the watermark should appear