You may come to a point where you think it would look a lot better with a browse button with textbox instead of just having a less-intuitive URL textbox.
Users never know whether its an absolute, relative to root or relative to web url.
So the object is: AssetUrlSelector
You can find it in
You might need to manually browse for the Microsoft.SharePoint.Publishing.dll reference in
C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\12\ISAPI
Note: This will only work with MOSS 2007 and not WSS3 since the dll will not be available.