use as
<marquee onmouseover="this.stop();" onmouseout="this.start();" SCROLLAMOUNT=5>
<asp:image id="Image3" runat="server" height="100" width="80" imageurl="~/images/laptop.jpg" xmlns:asp="#unknown" />
<asp:image id="Image4" runat="server" height="100" width="80" imageurl="~/images/galaxy.jpg" xmlns:asp="#unknown" />
</marquee>