..and we need to access it:
ScriptManager ScriptManager1 = (ScriptManager)Master.FindControl("ScriptManager1");
ScriptManager1.AsyncPostBackTimeout = 300;
..or a much beter way:
ScriptManager ScriptManager1 = ScriptManager.GetCurrent(this);
suggested by rajbk
Not sure if this is the best method, but this is the one I used to preload the image.
<script type="text/javascript">
if (document.images)
{
imgPreload = new Image();
imgPreload.src = "../Images/ajax-loader.gif";
}
</script>