As with the whole premise of the Entrecard credit/advertising network, it is important to remember that your "125x125 pixel ad" does more than simply promote your blog. Similar to traditional advertising, you must appeal to both your visitors, other bloggers on the network you are hoping to advertise to, as well as visitors to other bloggers (who may or may not be members of the Entrecard network).
The main ideal to remember is that your advertisement should make a bold statement about your business, website, or blog (your preference as to what you want to call it).
When designing your ad, think about the whole purpose of your site, and include some element of this on your advertisement. For example, if you focus on your personal life, a caricature, personal photo, or other representation of yourself should ideally be placed on your advertisement.
Secondly, other members may prefer to see some type of words on the advertisement, at least until you have branded your site/advertisement to the point that anyone can recognize the image that is displayed (perhaps corresponding to the color scheme/layout/image a part of your website template). These should include your main site title, a phrase or tagline about your site, or your name, again if you are blogging on a personal domain using your name.
While you can always stick with the default advertisement (using a text/image combination) provided by Entrecard (as I have done), it is always better to produce your own variant of an advertisement. If you do decide to use the default Entrecard ad/widget, make sure that your tagline and title combination are catchy, and not a run-of-the-mill title and uncreative slogan.
Another final point to remember is the fact that you are trying to get the most from this advertisement, whether you advertise your "card" on other blogs, simply use it to gain credits and/or traffic by dropping, or through other promotion methods through the system. It must be unique, creative, and eye-catching, or you won't be able to leverage the amount of traffic that you had intended on.