Now my system won't built. It appears to have a problem with png files.
Why would that prevent the system from booting correctly?
Not enough information provided about your problem.
I assume you have enough HD space, memory and can boot but
are not getting an X session.
Did You do a shutdown -Fr and the system doesn't boot into X?
Sometimes a kernel hasn't installed correctly and applications fail.
Make sure the latest kernel is selected for boot or uninstall then
reinstall the lastest kernel.
$ rpm -qa | grep -i kernel #lists kernels
$ rpm -e lastkernel #to remove the latest kernel
$ up2date -install lastkernel #reinstall the lastest kernel
Look for error messages
$ less /var/log/boot.log
$ dmesg
$ less /var/log/messages
$ less /var/log/Xorg.0.log
Worse case reinstall FC3 and rerun up2date.