as you are using a publishing site, isn't it easier to add an AssetPicker?

Would be something like this:
//Declaration
private AssetUrlSelector assetUrlSelector;
……

In the CreateChildControls() do the following:

base.CreateChildControls();
this.assetUrlSelector = new AssetUrlSelector();
assetUrlSelector.ButtonType = ButtonType.Image;
assetUrlSelector.PickerButtonImage ="url to image";
assetUrlSelector.ToolTip = "tooltip";
assetUrlSelector.DefaultToLastUsedLocation = false;
assetUrlSelector.DefaultOpenLocationUrl = "url to defaultlocation";
this.Controls.Add(assetUrlSelector);

If you want to link to the selected site do change the following in the XML (and maybe some more adjustments in the code):
URL => Must be Capitals!!!!!

Greetz, Verhaad