It does take a little bit of HTML know-how, but the complexity also depends on the layout you're using and what exactly you're attempting to change. For example, if you're using the built-in layout "Background", you can simply press the "Properties" button in the Screen section of the Layout tab, and change the background image there. Each cell can also have its own background image, so if you're not trying to change the entire screen's background image, but only a small part of it, the procedure would be a little different. What layout are you changing?