Have you tried a Google search of the image?  Sometimes this is called a reverse image search.  There are different methods to do this: https://support.google.com/websearch/answer/1325808.  May not get anything, but it's worth a shot.

Ok, I had some spare time this morning and I love an interesting rabbit hole to fall down in research because a basic reverse image search came up with nothing.

http://2.bp.blogspot.com/-D7ZS4wZ9o60/U71wxBLdWNI/AAAAAAABgNE/ilATH1sjESM/s1600/...

I found it in a book called "Armies of the Macedonian and Punic Wars 359 BC to 146 BC" by Duncan Head with illustrations by Ian Heath which is evidently a wargaming research book from the late 80s

If you want to use it for anything other than non commercial personal use I would say contact Ian Heath and find out who owns the rights.

I did find a white background version and cleaned it up though so I will include that.

Awesome find Kordwar!