Fix images for pages not in root (/phobos)

Fixes issue 9544.

76e0de544f9d0aa67bbcad9127cd320dbdb7cc80 broke images for pages not in the root of the website (most notably /phobos pages).

Not having local images work has always bugged me too but it's more important that the actual website have working images.

@WalterBright and anyone else: A quick way to test locally with working images is to just run `python -m SimpleHTTPServer` from the root directory of this repository. Just navigate to http://127.0.0.1:8000.