Advertise for book cover design?
when I can advertise for my web site that design cover for books,please offer some online method
That's quite a specialty, you might want to buy web ads that target specific sites or pages that authors frequent, there are a number of ad systems like Google Adwords' Ad Network that allow you to pick the specific publisher's site where your ad would appear.
Certainly you would want to use a postal mail campaign targeting the relatively small number of publishers out there.
There are various freelance designer sites where people look for designers, like odeslk.com, elance.com, the site 99designs.com allows you to submit a draft design to a design contest a client sets up, Book Covers are one of their categories that probably has a fixed price.