I found a really simple on-line software application bannerfans that makes banners. It is no Photoshop, but can be used to create a simple banner for your website / web page. To get the image above, I created a banner in the site and then once completed, the site gave me the option to insert the provided html code into a website or blog. (For your info – BannerFans uses imageshack to store the created images/banners). (The image used of the various famous buildings of London I purchased from istockphoto).
For more professional and stylish banners or banner adds. I would recommend Adobe Photoshop. Its probably the best tool for creating banners and banner adds. If you want to create an animated banner add. Then Flash is probably one of the best tools for this. There are numerous software tools for creating banner adds and another really good one is Adobe After Effects, where video compositions and animations can be saved as flash files and used on the web.