Opera needs dep =media-libs/libpng-1.0.12 (not higher) --> when installing libpng-1.0.12, X has trouble -> all icons in KDE are gone.
When deleting libpng-1.0.12 or the newer libpng-1.2.1-r1 (by emerge unmerge ...), kdm fails when starting.
When both are installed, the following lines can be found in /var/log/kdm.log:
libpng warning: Application was compiled with png.h from libpng-1.2.1
libpng warning: Application is running with png.c from libpng-1.0.12
libpng error: Incompatible libpng version in application and library
solution (important that you do it -in this order-)