Image with runat=server cannot see code-behind property
<p>I've got the following in my .aspx:</p>

<pre><code>&lt;input type="image" src="&lt;%=PayPalButtonImage %&gt;" onserverclick="RedirectToPayPal" runat="server" /&gt;
</code></pre>

<p>In the code-behind I've got this property:</p>

<pre><code>protected string PayPalButtonImage
{
    get { return PayPalExpressCheckoutButtonUrl;}
}

protected void RedirectToPayPal()
{

}
</code></pre>

<p>why can't it see this property or the server method RedirectToPayPal?  I get a runtime error of :</p>

<p>'ASP.cart_aspx' does not contain a definition for 'RedirectToPayPal' and no extension method 'RedirectToPayPal' accepting a first argument of type 'ASP.cart_aspx' could be found</p>
