Generally, I only see image issue now due to misconfiguration of the
wp-config.php file or the server not being configured with an image editing library. WPMU, thats a different beast…
The numbers should be the site ID number. The site that you have Connections installed in, which is that site ID? 27 or 28? I suspect since Connections is looking for 27 that the site ID is 27.
Were there any other folders in the uploads/sites/28/27 folder beside the connections-images folder?
Can you please add a new test entry with an image? Where is the image saved? In the ploads/site/27 folder? And does it display fine or is an error generated?