I've had a few people looking at banner ads for the Spectrum Website.   One of the common questions I've had is how to design the banner ad to be the most effective.  There is a lot of information on the web that can be used, but I figured I'd list a few resources that I recommend.

Articles: 

The Complete Guide to Creating Effective Square Banner Ads
http://www.anywired.com/the-complete-guide-to-creating-effective-square-banner-ads/146/

Banner Ad Creation Software

For easy of use, and the most options available, without having to be graphic designer, I have found Websylte 4 to work best.  I've tried a few other banner creating software, but they always seem limited.

Webstyle 4
http://site.xara.com/products/webstyle/

Of course, working with a graphic designer will always get you a better product, but you really need to know what you want ahead of time.