What I've done:
- i've made changes in latest timthumb.php (v1.25) - originally Network theme comes with v1.19
- now process also pictures from unlimited external domains
- disable verify_url function (pattern has bug, so some url want process)
- now both WP and WPMS/BP installation use /uploads/cache folder to cache files
- new function getImage($num,$content) at the end of file - get url image from post
* $num - which picture (is post have several pictures) - default is 1
* $content - post content
- small modification of functions multisite_recent_posts and get_recent_posts to call getImage function
- market but it can activate if you need - check that picture exist on url before to show like thumbnail (slow if you use lot of features blog on your home page) - this is because some post have broken links to images (user mistakes).
Maybe this changes will be for someone helpful.
You can check at http://blogge.rs how this thing works.