Thanks Erel - Works great.
Solution, hope others can benefit:
B4X:
Sub tmrBatt_tick
showtime
End Sub
Sub showtime
ImageIndex = ImageIndex + 1
If ImageIndex > 4 Then
ImageIndex = 1
End If
Dim bd As BitmapDrawable
bd.Initialize(LoadBitmap(File.DirAssets, "Charge" & ImageIndex & ".png"))
bd.Gravity = Gravity.FILL
imgCharge1.Background = bd
imgCharge1.visible = True
End Sub