During restore all backups, image and file backups, behave like full backups, so you don’t have to care about that at all. The difference is what is transfered during the backups.
An incremental file backup only transfers the files changed since the last backup. A differential file backup would transfer all the files changed since the last full backup. Which would of course be sub-optimal.
But you are right: The image backups should be called differential, because there every image is based on the last full. This is done so the server can delete any incremental image backups, not just the last one in the chain. See here for the difference http://www.acronis.com/resource/solutions/backup/2005/incremental-backups.html .