A few good spots for general html5 learning resources are html5rocks html5gamedevs and developer.mozilla.org. Also w3schools is a great html/javascript reference resource.
To generate a base64 string from your image you can use an online "base64 image encoder".
Then to bake the string into css you'll use a data url like this:
div#backgroundDivId { background: no-repeat url("data:image/png;base64, ...."); }
That way the css and images would get loaded as one before the game starts.
And take heart, as you work on and release projects, you'll only get better and better.