Workaround: use a bigger image 
Me.radCheckBox1.ButtonElement.CheckMarkPrimitive.CheckElement.ShouldPaint = False
    Dim dpi As Point = NativeMethods.GetSystemDpi()
    Dim scale As Integer = dpi.X
    If scale >= 300 Then
        Me.radCheckBox1.ButtonElement.CheckMarkPrimitive.Image = Image.FromFile("..\..\check-mark.png")
    ElseIf scale >= 200 Then
        '...
    Else
        '...
    End If