Many webmasters use "button" banners to advertise their websites. Buttons are square graphics and normally measure 125 X 125 pixels. Larger versions (150 X 150) are also used, but 125's are the most popular.
Whether you create your own button or have one professionally made, you really need to consider its primary purpose before finalizing the design. The purpose normally is to grab the attention of a shopper, and entice them to click to your web site.
Most buttons you see are small versions of a logo graphic which often display the site name, a cute graphic, and use earth tones. Most are attractive, but do they really pique the interest of a shopper?
If you were decorating a store window, would you just show your business name and graphics, or would you display a few of your most popular items?
More information concerning advertising graphics (including designers) can be found on our Craft Site Medic's Graphics Page. For a list of great locations to advertise your site or blog, please visit our Webmaster Resources Page.