A picture is worth a thousand words, but be sure you say what you mean with your images. If you are going to use an image it can save you lots of text on a banner. For example, a couple credit card icons show that you accept credits cards and have something to sell online. Add an image of a computer, and you show yourself as an online computer hardware merchant, without using any words. Free Software and Hardware offers get the higher responses, while free travel and money offers rank much lower.
People are more likely to click on a new banner than one they've already visited.
Be sure that your banner looks clear and readable.