libpng errors
Problem
Some changes in libpng version 1.6+ cause it to issue a warning or even not work correctly with the original HP/MS sRGB profile, leading to the following stderr:
libpng warning: iCCP: known incorrect sRGB profile
The old profile uses a D50 whitepoint, where D65 is standard. This profile is not uncommon, being used by Adobe Photoshop, although it was not embedded into images by default.
Solution
The simplest solution is simply to remove the embedded profile from your image. This can cause a slight shift in color if you have a properly calibrated system, monitor, and software. If you really need it (say for a print shop), you can alternatively embed a different color profile. If this applies to you, you probably have the profiles you need already.
GIMP
To remove the embedded profile, go to the menu Image > Mode > Assign Color Profile and set it to RGB workspace(sRGB built-in).
To change the embedded profile, go to Image > Mode > Convert to Color Profile where you can choose a profile you already have loaded or load a new one from disk.
ImageMagick
To remove the embedded profile, just run:
$ convert -strip input_filename output_filename