Fix png images causing libpng 1.6.3 to print warnings.

Hi.
The [libpng](http://www.libpng.org/pub/png/libpng.html) i use, version 1.6.3, (on Fedora 20) complained about these files.
The warnings were:
```
libpng warning: iCCP: known incorrect sRGB profile
libpng warning: iCCP: profile 'ICC profile': 'RGB ': RGB color space not permitted on grayscale PNG
libpng warning: iCCP: profile 'icc': 'RGB ': RGB color space not permitted on grayscale PNG
```

As far as i see they are rather harmless, the old versions of the images where displayed OK.
But getting the warnings as console output is quite annoying.