Sorry to be a pain but put a ruler on the end of your nose (for accuracy, stick it in your eye) and touch the other end to a mirror. You may have to adjust yourself a bit but you will see two rulers, end to end, from you to you'. One of them is in your hand and the other one is an image (and also has your "hand" holding it) but it still has the property of measurement.
The image of yourself that you see "in" the mirror isn't actually "in" the mirror. The rulers are showing you that the mirror is doing something that causes an image of you to appear exactly twice the distance from you to the mirror.
The mirror image of yourself is actually on your own retina, which is exactly the distance from you to the mirror and from the mirror to you.
We can debate semantics and to be fair, most people will use your terminology. It's bloody hard to ignore the "evidence" of your own eyes!