Does CyanogenMod Installer create a recovery image? I'm looking at installing CyanogenMod onto my Galaxy S3 (i9300) and the CyanogenMod Installer looks to be the easiest way to do this.

However I would like to get an image of my phone before I attempt to install CyanogenMod. This is so that if anything goes wrong (or I dislike the ROM) then I can just revert back to an image of the phone as if nothing had changed.

Google searches suggest I need to root my phone, unlock the bootloader and then install ClockworkMod Recovery to create a backup.

If I do go down this route then the automatic rooting and bootloader unlocking that the CyanogenMod Installer will do seems completely redundant, since I'm going to have to do it manually anyway to get an image.

That doesn't really make sense to me - so can anyone please advise on what I should do to get this image before I start?
Your google research is correct. There is an order. root → unlock bootloader → install custom recovery (twrp or cwm) → backup current rom → install custom rom (cyanogenmod or aosp etc). Without being rooted and having bootloader unlocked, you won't be able to install recovery and custom room. That's why it needs to be done first. Cyanogenmod is pre-rooted. So you don't have to do it again once it's installed. Initial rooting of your current rom and installing recovery is only one time process. You can then backup/restore the roms using the recovery of your choice (twrp/cwm). Assuming you have rooted/installed custom recovery, make sure to backup current rom should anything go wrong. Also wipe data/cache/dalvik cache before installing the new custom rom. There is usually a hardware combination button to access recovery menu or you can issue reboot recovery command from terminal.