You have only a couple of seconds to grab a reader's attention, and a great image is often the most effective way to do it.
When people look at ads, almost all of them look at the image before they'll even think about looking at the words. However, the image must relate to what your selling, otherwise you just create confusion and people won't bother to read your ad.
The image should be custom made to reinforce the words in your ad. If you don't, you risk wasting your entire ad budget. If you're selling a product it should also create desire.
