image burn is for writing CD DVDs
Bootable image of the system really means a clone of the drive.
Acronis will take an image of your drive / partition and compress it to an external drive this image can be restored back to the origina drive and it shoul then be bootable or it can be restored to a new drive and again be bootable acronis also allows yo to extact (copy) and use file from the image however the file once changed is not put back into the image.
I take an image of my system drive C: and data drive F: every Friday night, to an external drive, using two different names for the images. This leaves me with the image from last week and the week before. Any important documents I'm working on get backed up immediately to the same external drive