Every MUI object has its own background setting. The background is displayed "behind" the actual object contents, e.g. behind a the text of a text object or behind the image of an image object.
An object without a specific background setting will inherit the pattern from its parent group. The default background for a window and many other background patterns are adjustable with the preferences program.
Please note that it is generally not a good idea to change the background to a hard-coded image because this can easily clash with the user's current skin settings. Remember that everything in MUI is user-configurable so you should not override these settings by defining your own skins.