The below ought to get you started.
Within the aspx page enable page methods within your ajax script manager.
<asp:ScriptManager ID="ScriptManager1" runat="server" EnablePartialRendering="true" EnablePageMethods="true" />
Template column below. The commented out image was using onmouseover which is what you're looking for. The example below displays a image to be clicked on to display the details.
<asp:TemplateField>
<HeaderTemplate>
<asp:Image id="imgLineItems" runat="server" ImageUrl="/Images/LineItems.gif" AlternateText="" ToolTip="PO Details" />
</HeaderTemplate>
<ItemTemplate>
<%--<asp:ImageButton id="Attachments" runat="server" ImageUrl="~/Images/clip.gif" commandArgument='<%# Eval(Container.DataItem, "JobID") %>'
commandName='<%# Eval(Container.DataItem, "POID") >' OnClientClick="return false;" onmouseover="$find('pceAttachments').showPopup();" onmouseout="$find('pceAttachments').hidePopup();" />--%>
<asp:Image id="imgLineItems" runat="server" ImageUrl="/Images/LineItems.gif" AlternateText="" />
<asp:Panel ID="pnlLineItems" runat="server"></asp:Panel>
<asp:PopupControlExtender ID="pceLineItems" runat="server" DynamicServiceMethod="GetLineItems"
DynamicContextKey='<%# Eval("POID") %>'
DynamicControlID="pnlLineItems"
TargetControlID="imgLineItems"
PopupControlID="pnlLineItems"
Position="right">
</asp:PopupControlExtender>
<asp:DropShadowExtender runat="server" ID="FileDownloadDropShadow" Opacity=".25" Width="2" TargetControlID="pnlLineItems"
TrackPosition="true"></asp:DropShadowExtender>
</ItemTemplate>
</asp:TemplateField>
Code Behind:
System.Web.Services.WebMethod()> _
Public Shared Function GetLineItems(ByVal contextKey As String) As String
Return BOs.POLineItem.LineItemsPopUp(contextKey)
End Function