Use SystemParametersInfo API function

DECLARE INTEGER SystemParametersInfo IN user32; 
	INTEGER uiAction, INTEGER uiParam,; 
	STRING @pvParam, INTEGER fWinIni

with desktop parameter

#DEFINE SPI_SETDESKWALLPAPER 20

Check MSDN page for this function
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/sysinfo/base/systemparametersinfo.asp

FoxPro code sample:
http://www.news2news.com/vfp/?example=283&function=441