I did some research and I am trying to add the background image but it doesn't display. Anyone knows how to fix this? Here is my code: 

Protected Sub Checkboxlist1_DataBound(sender As Object, e As EventArgs) Handles Checkboxlist1.DataBound
        If test <> "" Then
            Dim checklist As List(Of String) = ReturnListofStringClass.ReturnChecklistListOfString(test)

            For i = 0 To Checkboxlist1.Items.Count - 1
                For Each id As String In checklist
                    If Checkboxlist1.Items(i).Value = id Then
                        Checkboxlist1.Items(i).Selected = True
                        Checkboxlist1.Items(i).Attributes("style") = "background: url(http://www.example.com/assets/img/checked.png) no-repeat red; height:300px; color: blue; font-size:20px;"
                    End If
                Next
            Next
        End If
    End Sub

Thanks for your help.