require 'Win32API'

SPI_SETDESKWALLPAPER = 20
SPIF_SENDCHANGE = 0x2
SOURCE_FOLDER = "C:\\Users\\Gaurav\\Pictures\\Best\\"

files = Dir.entries(SOURCE_FOLDER) - [".", ".."]
file = files[rand(files.length)]

systemParametersInfo = Win32API.new("user32","SystemParametersInfo",["I", "I", "P", "I"],"I")
p systemParametersInfo.call(SPI_SETDESKWALLPAPER, 1, SOURCE_FOLDER + file, SPIF_SENDCHANGE)