The article you mentioned is intended for Forms 1.0, our old editor - customers who started with Marketo this year don't have access to it because we're trying to slowly phase it out of use.
This article walks through how to customize the form button style and color in Forms 2.0:
You might also consider using some of the resources we have here to further customize it:
like example #5, which explains how to make the form submit due to a click event on an element that's not part of the form, and then hide the form button using CSS (it has the class "mktoButton").
Thanks Grant. This seems be more difficult than necessary. The customizations to the button in the form editor are not ideal, as they are extremely limiting. Seems like a pretty basic function, to use a custom image instead of the ones provided by Marketo. Almost every email provider I've ever used allowed this.