How to show all online users with the online image

Just edit the ItemTemplate of the ListView and bind the ImageUrl of an Image control to a field that says if the user is online or not:

<itemtemplate>
            <tr>
                <td>
                    <asp:image id="isOnlineImage" runat="server" imageurl="<%# ((bool)Eval("IsOnline")) ?"online.png":"offline.png" %>" />
                </td>
              <td>
                    <asp:label id="UserNameLabel" runat="server" text="<%# Eval("UserName") %>"  />
                </td>
               
            </tr>
        </itemtemplate>

In that code I am assuming you have a field called IsOnline, and two images online.png and offline.png.

You can get the idea from that an adapt it to your logic.

hope it helps.