Reference current element's height/width

Is there a way in CSS to refer to the width of the current element?

I would like to position an image to the left of another image using margin-left: -width but can't seem to find a way. The image is not a fixed size but it is half the paren't height, if that helps.

I found one other question asking something similar but that was 2+ years ago, before calc() was introduced, and I thought maybe there would now be a way to do it.