If you are using the custom CSS you will probably need to use more images that are not called in the interface. There is a technique where you can put your own images.
You must know that all images that you put in the images folder of your template will be removed as soon as you will update the template from the interface.
Put the images in a custom folder
Go in your template folder and create a subfolder. You can name it as you want, in our example we will call it custom
The folder will then looks like
ROOT/templates/YOURTEMPLATE/images/custom
Use the images path in your custom CSS
When writing your custom css you just have to point your images to the folder that you have created. Example with a background :
#test {
background: url(../images/custom/myimage.jpg);
}
Note that you an organize your images in multiple subfolders if you want