First: audio does not display artwork, only video, except for the special case of m4a. m4a is not an HTML5 compatible format either. HTML5 audio embeds look different in each browser/platform, it's outside of our control without including JavaScript, which adds a lot more complexity.
Second: Please reply with a link so we can see. Also make sure your iPad is up to date, the first versions did not support HTML5 and was added a few months after the iPad was launched.