<asp:Button ID="tianjia" runat="server" Text="" Visible=false
           style=" background:url(/images/dincan/an1.jpg); ; height:30px; background-position: center center; border:0px; cursor:pointer;" 

onclick="tianjia_Click" OnClientClick="return check()" />

OnClientClick="return check()"

<script type="text/javascript">

function check() 
{ 
     alert(‘ceshi‘); 
     return false; 
}

</script>