Default Can't modify priv-app apk!!help

1.firstly has signing system apks changed in kitkat bcoz i cant run my modified apps on my phone? is my problem,whenever i modify the file and sign it and replace them with those of system priv-app the apps disappear and when i replace them with original ones they still dont work!!!
3.i only changed some icon pngs
If i helped you ,you know where the Thanks button is!