so the $img_ps_dir might predate 1.7.8
I instead tried <meta property="og:image" content="/img/metacardlogo.jpg" /> in the theme/classic/templates/_partials/head.tpl inserted after line 65.
and that did in fact overwrite every other og:image throughout the site, as i suspected it would, so I assume that I would need to wrap the meta property in an {if !isset} to detect if ONLY it is the root URL, but I am at a loss of how to properly do that.