I prefer to pull the URLs from a tags 'href's or form 'action's - or some other element that makes sense.
<img class="_click" src="<%= Url.Content("~/my/image.png") %>" alt="Click" />
And in my javascript (double check this on jQuery, I am not sure if it's the exact syntax.):
var url = $('._click').attr('href');
// To pass these to your plugin as options
// see lightbox source for a full list
$('a').lightBox({ imageLoading : url })
Another slightly less preferred option is to add your settings on top of your file:
<script type="text/javascript"><![CDATA[
$('a').lightBox({ imageLoading : <%= Url.Content("~/my/image.png") %> })
//]]></script>
I said 'less preferred' because this approach mixes markup and code.
Yet another approach (which needs a lot of tidy up) is to serve you js file from a controller:
public ActionResult GetFileContent(string filename)
{
// important: make sure to control the path for security
var path = Server.MapPath("~/Scripts/" + filename);
var content = System.IO.File.ReadAllText(path);
// Use some kind of template convention
content = content.Replace("{{IMAGE_PATH}}", Url.Content(@"~/my/image.png"));
var bytes = new UTF8Encoding().GetBytes(content);
return new FileContentResult(bytes, "text/javascript");
}