When hovering the mouse to Confluence images in a webpart one can see an extra slash '/' character. For example:
http://<your confluence url>//s/1627/49/_/images/icons/docs_16.gif |
One can see this behavior if the Confluence URL set in the WebPart's Confluence setting has a trailing slash:
http://<your confluence url>/ |
Go to your Webpart Confluence setting and remove the trailing slash.