It depends on whether your game will be:
1. Playable on-line (html & media files are hosted on the same web-server)
2. Playable on-line (html & media files are hosted on different web-servers)
3. Downloadable & Playable on the end-user's local machine.
In use-cases 1 & 2 it makes sense to also host the media on-line, and in use-case 3 it makes sense to store the media in a loca location that is relative to the HTML file.
Also if setup correctly it is possible to use the same Relative URL to access the media in use-cases 1 & 3.
eg. assuming the media is stored in a media folder on either the web-server or the local machine then...
... could be used to access the above image in both situations.