What are the default variable names in Latex for the width and height of an image?

I have many images - typically named \my_image_m_n - and I wish to print-off the width and height in mm of each one of them. How can I get LaTeX to print off all of these dimensions? Intuitively, one might think these dimensions would be held by variables with names like: \imagewidth and \imageheight. But, try as I may, I cannot get any sensible results using \includegraphics{..full path and file name} followed by \showthe\imagewidth. Are these names the correct names for the width and height of an image?

With your description I can only guess that you want to measure an image like:



You can also make it work more automatically:



they are not stored, if you want them, declare a box then use