I'm not sure how to help without looking at your project, and I can only look at it as any player would (you'd have to contact support for more). It sounds like you're correctly uploading the game proper, and setting it to be played in the browser, so:
- the assets are in the same archive, right? just making sure, because making (ZIP) archives confuses some people;
- if yes, double-check the file paths in your game. They should be relative to the game file. E.g. if you have a file called "cover.jpg" right next to "index.html", it should be referenced in game as "cover.jpg", with nothing else in front. If it's in a directory called "assets", then it should be "assets/cover.jpg", and so on.
Hope this helps.