You are right - CSS files for custom skins must be registered manually, unless you are using ASP.NET themes, as
Waleed pointed out. The opportunity to register custom skins manually exists so that developers who want to optimize their CSS, can combine all skins into a single stylesheet.
With regard to the images, RadGrid and RadCalendar offer the ImagesPath
property, which specifies a single location of all images the respective control needs, so that you don't have to declare a path for each image separately. The only thing to keep in mind is that in this case the control will search for images with specific names, listed here:
RadCalendar - Creating Custom Skins
the Telerik team