This example lets you find the current Desktop object and sets the current lockup background to BACKGROUND_TILED_IMAGE and BACKGROUND_IMAGE.
WPDesktop *Desktop;
Desktop = _wpclsQueryObjectFromPath("<WP_DESKTOP>");
_wpSetLockupBackground(Desktop,
"?:\\os2\\bitmap\\warplogo.bmp",
BACKGROUND_TILED_IMAGE,
1,
BACKGROUND_IMAGE,
CYAN);