Hi,
the error is caused by the scale_*.png images in your page. It seems that the images violate the PNG specification but still can be displayed in some software. Re-saving the PNGs could fix some problem.
Technical info:libpng reports "Too many IDAT's found". This can be reproduced by running e.g. pngcrush
$ pngcrush -q scale_aa.png
While converting scale_aa.png to pngout.png:
pngcrush caught libpng error:
Too many IDAT's found
A post about this error on graphics.png.devel:
http://old.nabble.com/libpng-cannot-read-a-valid-PNG-image-that-other-apps-like-MSPAINT-can-read.-td13639432.html