You can use ToggleButtonExtender from Ajax Control Toolkit and use images.
<asp:CheckBox ID="chkMonM" runat="server" AutoPostBack="true" OnCheckedChanged="chkMonM_CheckedChanged" />
<cc1:ToggleButtonExtender ID="chkMonM_ToggleButtonExtender" runat="server" CheckedImageAlternateText="Select" CheckedImageUrl="~/images/good.png" Enabled="True" TargetControlID="chkMonM"
ImageWidth="30" ImageHeight="30" UncheckedImageAlternateText="Deselect" UncheckedImageUrl="~/images/bad.png">
</cc1:ToggleButtonExtender>