I assume image should be the same size your game res is or higher
Based on what I was seeing on other game pages, that's the answer I was starting to lean to. Every example I saw had the embed BG image resolution at the same resolution or a larger resolution than their game.
Without having an html embeded game to test for myself, I couldn't say with 100% certainty that was the answer.
Glad to hear that you got it resolved!