Hi A.Samarian,
Put the image file in the same folder as the skin. I would put anything about images in styleimages.css
I don't know a good way to put a backround image in ALL Htm Content Modules without also putting it in the backround of other modules.
If you put the background image on the
.modulecontent{background-image: url('website_background.jpg'); }
this will put the image in the backround of all modules not just the Html Content module.
If you want to put an image in the backround of 1 Html Module then open it in the editor and click the View Source button in the editor toolbar, add a div with a custom css class.
<div class='mycustomclass'>
put content inside here
</div>
.mycustomclass{background-image: url('website_background.jpg'); }
Hope it helps,
Joe