Andrea & I have had several requests to add post thumbnails to Sitewide Tags. A couple months ago, Christine, emailed me an implementation she had done to get it working. I hadn’t had time to look into it until yesterday. I checked in a variation on what Christine had done earlier today. You can download the development version to test it.
There is a checkbox in the sitewide tags options in network options to turn on the thumbnail support. If the thumbnails do not show in your tags blog, you may need to add the template tag to the theme
<?php the_post_thumbnail(); ?>
I did not test with MU 2.9.X, but I suspect this will only work in WordPress 3.0 or higher.