@matthiaskrgr: That is our problem... If I understood correctly, libpng is 
saying that our color sheet (sRGB) is different from the one that it knows.

It's like saying that the color is blue but on libpng's libraries this color is 
green.

So it gets confuse and says that it is not recognizing the color, and that you 
probably used a modified version of this color.

Well, according to "libpng-config --version" command, my libpng version is:
1.2.46
So I cannot try to reproduce this bug.

This bug may be caused by different software versions... Or no?