Give @MajP 's suggestion a try. For example, change your above code to:
Me.Lvl1Img.Visible = Nz(Me.Lvl1Chk, False)
Me.Lvl2Img.Visible = Nz(Me.Lvl2Chk, False)
Me.Lvl3Img.Visible = Nz(Me.Lvl3Chk, False)
It may also be helpful if you could post a screenshot of your report in Design View here...