easier way to swap out thumbnail info? I hope so

Hope this is the right forum, if not let me know..

objective: to get a thumbnail to use for facebook meta tags.
specifically, I am trying to make this tag dynamic.
<meta property=”og:image” content=”http://greenspotantiques.com/images/my_fb_logo.jpg”/&gt;

I wish to retrieve a thumbnail URL to hand over to Facebook for each post page.
The problem: facebook wants images in a 50 x 50 format.
I’m in luck, I had already defined WP to generate 50×50 thumbs (by default I think?) — so all I need to do is figure out a way to “get” the url to that image.

the 50×50 image is NOT used in my theme, but it exists.
So I did the following….. but there’s gotta be a simpler and shorter method.

Logic: if it’s a single post page, I want to check if there IS a thumbnail, if there is, I need to read in it’s URL, and parse it out replacing the XX x XX tag with 50×50 and feed it out as a string, only IF the file exists. (in some cases it may not) – in that case, I’d feed out the my_fb_logo.jpg standard website logo…
(I’ve included the img src portion to test result, would be excluded in final).

help?
(reason for help? simple, this looks like a pretty hefty call as it would occur every time a page is loaded, in the header…. )
The original url being parsed out is in the form of
“/wp/wp-content/uploads/2010/08/th_GS_8-00889-1-270×225.jpg”

if(is_singular()) { //only for single posts or pages
$myID = get_post_meta(get_the_ID(), '_thumbnail_id', true); // returns IDentity number of thumb
$arr =wp_get_attachment_image_src($myID ); // to get the actual url of thumb
if(!empty($arr[0])) { // if empty bail
$str = $arr[0]; // first element is the URL, then sizes, etc
$myurl= (explode("/",$str)); // splits url into parts
$mybasehref= 'http://myserver.com'   .  $myurl[0] . '/' .  $myurl[1] . '/' . $myurl[2] . '/' .  $myurl[3] . '/' . $myurl[4] . '/' .  $myurl[5] . '/' ;  // results in URL without filename
$myimg=  (explode("-",$myurl[6]));  // extracts the filename

if(file($mybasehref . $myimg[0] . '-' . $myimg[1] . '-'  . $myimg[2] .'-50x50.jpg')) {
echo ('<img src="'. $mybasehref . $myimg[0] . '-' . $myimg[1] . '-'  . $myimg[2] .'-50x50.jpg' . '">' ); // test with img, to delete in final
echo ($mybasehref . $myimg[0] . '-' . $myimg[1] . '-'  . $myimg[2] .'-50x50.jpg'); // this would replace the "content" in meta tag for facebook.
 }
 } }

the result of the above is
http://myserver.com/wp/wp-content/uploads/2010/08/th_GS_8-00889-1-50×50.jpg
a working URL.