After publishing your game, click on the "Play" button.
That takes you to your game on
Is that what you were looking for?
If the file is inside your game, the link should be the same as the one for the game itself.
So if you play a game and it gives you an address like:
any images which are inside the game's zip will be accessible as:
This means that in most cases, you can use a relative URI (just the filename).
I've not really played with this, as I haven't got around to installing the desktop version yet. But I'm sure someone who has it can help you.