You should be able to do this just with CSS, no layout.master changes needed. Here's a starting point for you, and you can work from here with FireBug:
background: url("/Data/Sites/1/skins/ccds5-artisteer-24/images/blockheader.png") no-repeat scroll left center transparent;
I was able to get this going in just a few minutes of experimentation with FireBug.
I recommend you place your CSS changes in the style-artisteer-overrides.css file, not directly in the style.css file. That way they won't be lost if you make changes and re-export from Artisteer.