I think vscode should use the repository > directory field in package.json when generating the gif/image links in the html for their extension pages. Right now they just use the extension repository, so extensions not located at the root of a repo need to link to their repo's website if they want these to work both locally and on the marketplace.
For example, I thought hudson-trading/slang-server@bde3932 would have fixed the gifs not showing up, but it didn't.