Not sure which of these it is, but open up a shell and rename/delete ENVARC:Sys/wbconfig.prefs or ENVARC:Sys/workbench.prefs and reboot. That should restore the defaults.
The icon quality probably made you run out of Chip RAM. I've never found any benefit in using anything other than the lowest possible setting. Make sure your icons are stored in 'other memory' as well.
