Graham deadpixel
It would require some xtscript
this code would go in _headtags
on the main homepage (index} it would set homePage.jpg as the image
on other pages it takes the path of the page removes any dots and slashes and returns that as the image name IE /mp3/index.html would return mp3indexhtml.jpg as the image name
if your main index is index.html or something adjust the value in the 'if'

<!--parser:xtscript--> var $path = <xt:url type="path" noquery="1" /> if $path == /index var $ogImage=homePage else var $ogImage = call str_replace $subject=$path; $search=/ ; $replace var $ogImage = call str_replace $subject=$ogImage; $search=. ; $replace endif print <meta property="og:image" content="http://site.com/images/$ogImage.jpg"> <!--/parser:xtscript-->