This script will select a random picture (jpg) from a specified folder (default My Pictures), convert the picture to BMP and set it as the desktop wallpaper on next boot.

Set objFSO = CreateObject("Scripting.FileSystemObject")
Set objWShell = WScript.CreateObject("WScript.Shell")

strProfile = objWShell.ExpandEnvironmentStrings("%USERPROFILE%")

strSrcPath = strProfile & "\My Documents\My Pictures\"

intMaxLenth = 500
intMinLength = 1

Randomize  

intMax = Int(((intMaxLenth - intMinLength + 1) * Rnd))  

dirScan strSrcPath,0,intMax

Sub dirScan(dir, count, max)

If count > max Then
Exit Sub
End If

Set objDir = objFSO.getFolder(dir)
For Each objItem in objDir.Files

If NOT count > max Then

If objFSO.GetExtensionName(objItem.Path) = "jpg" Then
count = count + 1
End If

If count > max Then

setWallpaper objItem.Path

End If

End If

Next

For Each objSubDir in objDir.SubFolders

If NOT count > max Then
dirScan objSubDir.Path,count, max
End If

Next
End Sub

Sub setWallpaper(strSrcPic)

Set objShell = WScript.CreateObject("WScript.Shell")

strWallPath = objShell.ExpandEnvironmentStrings("%APPDATA%") & "\Microsoft\Wallpaper1.bmp"

objShell.Run "mspaint """ & strSrcPic & """"

Do Until Success = True
Success = objShell.AppActivate("Paint")
Wscript.Sleep 100
Loop

objShell.SendKeys "%fa"
objShell.SendKeys strWallPath
objShell.SendKeys "{TAB}"
objShell.SendKeys "22"
objShell.SendKeys "%s"
Wscript.Sleep 500
objShell.SendKeys "y"

Success = false
     Wscript.Sleep 1000
 Success = objShell.AppActivate("Paint") 

objShell.SendKeys "%{f4}"

objShell.RegWrite "HKCU\Control Panel\Desktop\Wallpaper", strWallPath

End Sub