By using the Value Display as opposed to the Image widget, your Mashup will be able to support the Service you've already written. If you prefer to use the Image widget, your Service simply needs to return an IMAGELINK of the following format: "Thingworx/MediaEntities/name_of_Media_Entity".
For the value display suggestion. It works.
For the Image widget suggestion,
I tried using var result = MediaEntities["Connection_Down"]; with result set to IMAGELINK. But seems it returned a broken link. Just wanted to find out more if the script is written correctly in this case...
Using REST way Thingworx/MediaEntities/Connection_Down renders the image successfully though.
Please advise. Thanks!