honestly I don't think you will be successful on this one.
If you look into the way Google transfers data you will notice that - when making a search - there is not a single image transferred. The search result - including images - comes back as a JSON object, which includes the images as base64 encoded binary data. Client side java script is rendering the incoming data into the DOM tree that is actually displayed in the browser and what you see when you click "View Source" or analyze the source.
Web Gateway does not see the same source, as it is rendered on the client, so applying the HTML opener will fail as the tags don't exist. Even blocking images by media type might fail, as no images as such are going through the proxy.
Such techniques are used on some web sites which have a huge number of visitors, for example Facebook does a similar thing. Even if the HTML Opener works on some sites it only works on the sites that come as full HTML source to the MWG. In Facebook this is only the very first request, so you can apply some HTML filtering here. As soon as you click a button or even refresh the data is obtained as optimized data that are rendered in Java Script.