Where to upload images & where to add custom CSS?

So I'm a bit rusty on HTML/CSS & new to this platform, so I apologize that this is going to seem extremely newbish of me

But when I'm uploading images into my directory to then use in custom CSS, what folder do I want to upload them to inside of the IPS directory?

And when I want to add custom CSS, for things such as adding an image to the Main body, which file should I edit/add this custom CSS to, or should I just create a new one for the custom things I'd like to customize?

Thank you!