This is a great video on adding banner ads complete with affiliate links to your WordPress blog. The author, Rajeev Kistoo, shows you how to add a banner to a widget from the WordPress admin area.
The only thing I wish the author had done differently here would have been to spend more time going over the importance of making sure the banner you select will fit into the space alloted to the wiidget.
Many widget areas within popular WordPress themes are no larger than 125 pixels. Adding an image to them larger than that will cause most all themes to “break” or in other words have display issues.
It can also often be difficult to determine the size of the widget area within some themes. Many list the sizes of the widget areas on the Widgets page as a part of the description of where the widget is located. Other themes are often a mystery as this information is often left outt of theme documentation.