Can Themes use pre-existing media folder already on site?

I am just learning WP and am “technically dangerous”. I have a folder on my site that I have hundreds, going to be 1000 or so, pictures in it. Some of them I will be using in WP. I am, right now, taking the time to actually try to do through tutorials (old age) having previously completely frustrated myself only to learn this isn’t desktop publishing! Anyway, I thought to myself that I shouldn’t be uploading media to WP which is already on the site…just point to the existing folder. I see that WP has it’s own folder for the media and when you point to another URL it still copies it into the WP folder.

Did I miss something in a tutorial? Is there something I should have checked off to avoid the duplication? And does WP automatically make three different sizes? Can’t it do that when needed?

Thanks.

Jim