I think you refer to “reimage” and not to “bare metal imaging” where the target computer is completely reinstalled from scratch.
You say that starting OS is Windows 7 with a single partition C:. Is it both system and boot partition or maybe you have an hidden partition that works as boot partition? (it generally does not have any letter assigned).
This can be seen with diskpart (e.g. select disk 0, list partition and list volume).
Reimaging a Windows 7 without dedicated boot partition using a Windows 10 setup image, only the system partition (C:) will be reimaged, other data partitions will not be overwritten.