The code would look like this
Sub BackG()
Dim L As Long
Dim strFolder As String
'note this will need to be ammended to point to your docs folder
strFolder = "C:\users\your name\Documents\Project-name/PowerPoint/Indesign_PNGs_for_ppt\"
For L = 1 To 20
ActivePresentation.Slides(L).FollowMasterBackground = False
With ActivePresentation.Slides(L).Background.Fill
.UserPicture (strFolder & "Background_slide__" & CStr(L) & ".png")
.Visible = True
End With
Next L
End Sub
NOTE ~/Documents/Project-name/PowerPoint/Indesign_PNGs_for_ppt IS NOT the full path to the folder and you will need to change the path in the code too.