could you go into a little more detail?

I was running into the same “System.Exception: Email should not be null or empty” error that Ezequiel reported above. In my case, the gravatar control was inside a repeater control on a page with a login control in the master. When a user logged out using the login control, I would get the error. The top of the stack looked like this:

FreshClickMedia.Web.UI.WebControls.Gravatar.Render(HtmlTextWriter output) +186
System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter) +27
System.Web.UI.Control.RenderControl(HtmlTextWriter writer, ControlAdapter adapter) +99

Here’s how I fixed it for me. My Page_Load looked like this:

protected void Page_Load(object sender, EventArgs e)
{
if (!Page.IsPostBack)
{
Session["Users"] = MembershipQueries.GetUsersWithRoles();
RepeaterUsers.DataSource = (IList)Session["Users"];
RepeaterUsers.DataBind();
}
}

So, the repeater with the gravatar wasn’t rebinding on postback (which apparently occurs when the login control is used to logout), and I guess that’s how the email ended up being null. When I changed my Page_Load to this, the error went away:

protected void Page_Load(object sender, EventArgs e)
{
Session["Users"] = MembershipQueries.GetUsersWithRoles();
RepeaterUsers.DataSource = (IList)Session["Users"];
RepeaterUsers.DataBind();
}

Hope that will help someone else.