If you're already using Easy Customization it's really easy-do you want it for the entire board? Just one forum? Either way, if you want it the easiest way to do it is via the EC. Log into your Admin CP and choose Easy Customization-the top URL input field is for a backgruond picture so once you've got it on your boomspeed account (Saw by viewing the properties on your avatar where it was hosted if you wanted to know
) and then assign the URL it has in there and save changes-if is the whole board you want it applied to that is where the check mark is by default-if you have any CSS in your template
you want to check the box, "Don't Clobber the CSS" to keep that CSS in the template.
Also if you haven't got the box checked at the top of the page to enable easy customization
it will save it but not show it-so make sure that is checked.