Not 100% sure what you are talking about but in IIS you have to allow "Directory Browsing" and make sure you don't include a file that is listed under the Documents tab. Both are options that you can get to in the properties of the folder as viewed through IIS console.
Maybe more effective would be to leave your index.html as is ... so that someone hitting your website sees your default page. Set up a subfolder where you allow directory browsing and put your images or whatever in there. Then put a link to that subfolder on your main page.
Does that make sense?