Devi flashare la recovery, come hai fatto con il root, il telefono deve essere in download mode.
Guida installazione ClockworkMod Recovery su Galaxy S
1) ClockWorkMod 6.0.4.8 (or higher) is required to install this ROM! Download: http://www.androidfilehost.com/?w=files&flid=15216 (you can also use Philz recovery which is also available at androidfilehost)
2) Download ROM .zip
3) Download Google apps (optional)
4) Put both .zip files on your device (either internal or external storage)
5) Boot your device into recovery mode
6) Make backup (if needed)
7) Wipe data/factory reset (not needed if installing updating from previous releases, but is necessary if upgrading between CM versions (example, cm-11.0 to cm-12.0 )
8) Install the ROM .zip file
9) Mount /system, install Google apps (optional)
10) Done.