isn't always as straightforward as many people think. To begin with, the best imagebanks (Like The ImageBank and Corbis) are very choosy - they don't accept images unless they meet strict criteria. You have to be very good to make anything at all out of it.
Starting your own image site is an option of course, and we'll provide as much help as you need. This whole question of images is fraught with problems - there are literally tens of thousands of people with websites trying to sell their images. It's very much a saturated market. I'm certainly not trying to discourage you, but it's as well to know what lies ahead. The pitfalls are many, but the main one is that people will simply take your images and use them without paying. There are ways of trying to prevent this of course, but all of them can be overcome by someone who's determined enough.
I suggest that you spend a good deal of time looking at Corbis
to get an idea of what they do and how they do it. The perhaps you can come back here with some more specific questions. Do you have a web-design program already, or something in mind?