Your meta tag should look like this:
" rel="nofollow" target="_blank">http://ia.media-imdb.com/rock.jpg"/>
And it has to be placed on the page you want to share (this is unclear in your question).
If you have shared the page before the image (or the meta tag) was present, then it is possible, that facebook has the page in its "memory" without an image. In this case simply enter the URL of your page in the debug toolÂ http://developers.facebook.com/tools/debug. After that, the image should be present when the page is shared the next time.