Hope I'm not too late...
From the image you provided I can see that you haven't unlocked the bootloader. It says rootable, where a unlocked bootloader should say rooted.
Anyway, if you want to flash cyanogen or hybrom etc., you have to have an unlocked bootloader.
It isn't really that hard to unlock it. And you can follow the official guide from sony ericsson to do it.
Regarding the flashing, don't worry. You just have to flash stock firmware.
Go to this link: [FIRMWARE][4.0.A.2.368][Mini][ST15i][Smultron] Official Firmware + ROOT solution - xda-developers
download the firmware, put it in flashtools folder under firmwares.
start flashtool, select the thunder icon, go to flashmode, select the downloaded firmware, click ok, wait until it asks to connect your phone.
Keep the -DOWN- volume button pressed while you're connecting the phone to the PC. The LED will turn green which means it's connected in flashmode.
It should flash it now....
After you flashed it, it should be back on .368 baseband, and should be rootable.
If you have any other questions or problems, just ask and I'll try to help you! :-)