If you want to display description in textbox on mouseover event on image button, then in that case you can hold the description in a hidden field.
<asp:HiddenField Id="hdnDesc" Value="Description goes here" />
Create a javascript method to show description on image button mouseover event.
<script language='javascript'>
function ShowImageDescription()
{
// get the textbox control in which description will be shown.
var textbox = document.getElementbyId(textbox_control_id);
var description = document.getElementbyId(hiddenfield_control_id).value;
if(description !=undefined)
{
textbox.value = description ;
}
}
</script>
call this javascript method on image button mouseover event.
<asp:ImageButton ID="imgButton" runat="server" onmouseover = "javascript:ShowImageDescription();" / >
This is how you can show description in a textbox whenever you move mouse over image.
----