I am using networked WP 3.2.1 (aka mu setup). I have the content directory set to "content" and not "wp-content" in the config. What is happening, is that the capture images are not being displayed because the URL for them starts with "wp-content" instead of just "content".
I have narrowed it down to one line in si-capture.php, in the get_captcha_url_si() function.
The URL is correctly set at first, until it reaches this line:
// "MU domain mapping" plugin compatabilty if ( isset( $current_blog ) && !empty( $current_blog->domain ) && !empty( $current_blog->path ) ) $url = (is_ssl() ? 'https://' : 'http://') . $current_blog->domain . $current_blog->path . ($wpmu == 1 ? MUPLUGINDIR : PLUGINDIR) . $si_dir;
Then it gets incorrectly over-written. Commenting out this line allows it to work correctly (I'm using it with job-manager).
Note that WP_PLUGIN_URL is correctly set, and so probably needs to be incorporated into this line, but I do not understand enough of the WP global variables to say exactly how, at least not without breaking other setups.