Kaz,
Sorry for the delay. Using the {SCOpenGraph image=xyz} tag is the right thing to do. When you set that tag though, it can take Facebook a day or so to 'see' the new tag and whatever was there until then.
You can use the
Facebook Debug Tool
to test your page and make sure the image is working correct. That usually will update their cached value as well, but the Send button can be a little sluggish to see the update.
If that still isn't doing what you're wanting, let us know the URL of a specific page you've set the {SCOpenGraph image=xyz} tag on and the Send button still isn't using it properly and we can look into it further.
Thanks,
Alex