<asp:Image ID="imgCrop" runat="server" />
your image have runat="server" so it may have different ID when it will be rendered.
so you need to have following in your page.
jQuery(document).ready(function ()
{
jQuery('#<%= imgCrop.ClientID %>').Jcrop({
onSelect: storeCoords
});
});
Apart from that you've visible="false" on your panel as shown in below code:
<asp:Panel ID="pnlCrop" runat="server" Visible="false">
So whole panel won't be rendered and your jquery code won't work. Rather use style="display:none", panel will get rendered and so the image will be and your plugin will work.