If the resource load is simply delayed it would eventually show up when the loading is complete. If the request is not dropped altogether is should show unless some custom CSS is overriding it (so please try disabling all your custom CSS for a while, just for test). This is why I asked you to make sure that it is returned properly and not a status 404 for example.
If you wish to preload the image from the assembly you can assign it as the ImageUrl of an image, for example:
sender, EventArgs e)
Image1.ImageUrl = Telerik.Web.SkinRegistrar.GetWebResourceUrl(
This will preload the default skin's button sprites. In a similar fashion you can load the other sprites, just copy the file names from the appropriate skin folder. Of course, the skin name should also match the skin you wish to preload.
If this does not help I strongly suggest that you send us a project that isolates this behavior so we can examine it and see what is going on.
All the best,
the Telerik team