IT WORKED!!

Thank you very much, Garibaldi99! You've been very patient and extremely helpful.
To deal with the problem of having to use only jpegs I added a function to check the file extension. If it is a jpeg then I go through the code you gave me. If it isn't a jpeg (i.e. if it's a bitmap) then I set the wallpaper normally.
Here's the code:
Sub Object_OnScriptEnter
Call checkday
End Sub
Function checkday
d= weekday(date)
Select Case d
Case 3
Call checkext
End Select
End Function
Function checkext
Select Case (object.PersistStorage("ext"))
Case "jpg"
Call LSetWallpaper(Object.PersistStorage("picpath"))
Case Else
system.SetWallpaper (object.PersistStorage("picpath")), 3
End Select
End Function
Sub Object_OnDropFiles(files)
Dim path
path= Split(files, "|")
newpath= LBound(path)
newpath2= path(newpath)
object.PersistStorage("picpath") = newpath2
object.ToolTipText= (object.PersistStorage("picpath"))
dropfile= Split(path(newpath), ".")
ext= UBound(dropfile)
object.persiststorage("ext") = dropfile(ext)
Call checkday
End Sub
Sub LSetWallpaper(jpgFName)
Set fso = CreateObject ("Scripting.FileSystemObject")
Set windowspath = fso.GetSpecialFolder(0)
winbmp = windowspath & "\wallpaperfile.bmp"
winjpg = windowspath & "\wallpaperfile.jpg"
If fso.FileExists(winbmp) Then
fso.DeleteFile winbmp
End If
If fso.FileExists(winjpg) Then
fso.DeleteFile winjpg
End If
fso.CopyFile jpgFName,winjpg,True
System.SetWallpaper winjpg,3
End Sub
Now, everything works! I can't thank you enough, Garibaldi, for everything. Hopefully, I'll upload my widget soon.