It depends what you called your template directory. If you called it my_template and this exists inside your includes/templates directory with all the related files and images, then you have done it right. However, if you have called it "bobred" then a folder should exist inside the includes/templates directory and inside this should be all the related images, css and files.

Assuming you have sorted out the folders and folder names, your css file inside the "bobred" template should point to the images within the images folder of the "bobred" directory.

Got that? Hope it is a clear enough explanation